C. B. Zeller

Whether or not the individual is a 5% owner for RMD purposes is determined only during the year in which they attain the applicable age. Let's say this person's applicable age was 72, which they attained during 2021. If they started the business in 2022, then they were not a 5% owner during 2021 because the business didn't exist in 2021. So they are not required to commence RMDs before their actual retirement, because they don't meet the definition of a 5% owner for RMD purposes.
Regarding the comment about the contribution being made after the end of the first plan year, there is a rule in 1.401(a)(9)-5(b)(2)(i) which says that you may determine the account balance on either a cash basis or on an accrual basis. So using zero is not incorrect, because you are permitted to ignore contributions actually made after the end of the calendar year.

This is correct.

It depends on whether they were a 5% owner in the year that they attained the applicable age. If they were a 5% owner at any time during that calendar year, then they must take an RMD. If they were not, then they can delay RMDs until their actual retirement.
Age 75 in 2024 means DOB was in 1949, which was the SECURE transition year. If the DOB was on or before 6/30/1949 then it is age 70-1/2 and the applicable year would be 2019, if was on or after 7/1/1949 then the age is 72 and the applicable year would be 2021.
Regardless, the 2024 RMD would be zero, since as you note, the 12/31/2023 account balance was zero.

This is to share with you the happy news that today is the 25th anniversary of the first day on which the BenefitsLink Newsletter began daily publication. I didn't see this coming when I decided to go daily in 1999, at age 41.
(The newletters had begun four years earlier, but they weren't being published every day.)
The free information must be helping employee benefits practitioners to help their clients, which translates to the ability of employers to effectively run and fund programs that improve the lives of so many millions of working people (and retirees, and beneficiaries), even if most of them wouldn't know (or want to know) the difference between an ERISA and an eraser. What a noble endeavor, to be an employee benefits practitioner!

Some lawyers and TPAs and other benefits practitioners have found work through our job board that's been running since 1996, which means they've gone to new workplaces and sometimes new cities, which means some of them have met people they wouldn't have met otherwise, which means some of them have fallen in love and then had children... which means there are people walking around on the planet now who wouldn't be here but for this "web site" thingie that started in 1995, and then the idea of sending "newsletters" by "email." None of that would have been possible without readers.

The existence of "BenefitsLink babies" didn't occur to me until one day about 10 years ago, but I kept it quiet -- at that time, they were still teenagers!

True to form, I and my business partner and wife Lois Baker (formerly an employee benefits lawyer, whom I met on CompuServe in 1990 while trading ERISA questions using dial-up modems) have failed to do any marketing of this happy day. But as I sat here at the keyboard today I had the idea that we would get so much joy by celebrating the occasion with readers. I hope this hasn't come across as a commercial but instead is the lifting of an E-flute of cyber-champagne -- here's to employee benefits practitioners everywhere! It's a wonderful community, and for 25 years now and still counting, we are so happy to be a part of it.

Yes, as long as you use the same definition of compensation for all participants.
You might also consider restructuring your test into component plans to avoid testing the young HCE on accrual rate.

Yes, as long as you use the same definition of compensation for all participants.
You might also consider restructuring your test into component plans to avoid testing the young HCE on accrual rate.

Assuming C has no employees, then
A: (2/80)/(1/22)=55.00%
B: (79/80)/(21/22)=103.45%
So plan B passes the ratio percentage test, plan A fails ratio percentage but might pass average benefits.
If A fails average benefits, then it would need to be aggregated with B for coverage.
Whatever aggregation options you use for coverage, you have to do the same for nondiscrimination (i.e. 401(a)(4) and ADP/ACP).
If A and B are aggregated for nondiscrimination, then A's safe harbor formula won't do you much good and you'll have to general-test them.

It depends on whether they were a 5% owner in the year that they attained the applicable age. If they were a 5% owner at any time during that calendar year, then they must take an RMD. If they were not, then they can delay RMDs until their actual retirement.
Age 75 in 2024 means DOB was in 1949, which was the SECURE transition year. If the DOB was on or before 6/30/1949 then it is age 70-1/2 and the applicable year would be 2019, if was on or after 7/1/1949 then the age is 72 and the applicable year would be 2021.
Regardless, the 2024 RMD would be zero, since as you note, the 12/31/2023 account balance was zero.

It depends on whether they were a 5% owner in the year that they attained the applicable age. If they were a 5% owner at any time during that calendar year, then they must take an RMD. If they were not, then they can delay RMDs until their actual retirement.
Age 75 in 2024 means DOB was in 1949, which was the SECURE transition year. If the DOB was on or before 6/30/1949 then it is age 70-1/2 and the applicable year would be 2019, if was on or after 7/1/1949 then the age is 72 and the applicable year would be 2021.
Regardless, the 2024 RMD would be zero, since as you note, the 12/31/2023 account balance was zero.

Looks to me like Plan A does not pass ratio percentage (I get 55%). If the CG can satisfy average benefits then you could test A separately on ADP and the PS appears to be a safe harbor (depending on comp definition).
If you do not pass average benefits then you MUST aggregate A with B to satisfy coverage and then must also aggregate to satisfy nondiscrimination (ADP, 401(a)(4)). 
Since B is cross-tested, you may very well have to run average benefits for the CG anyway, so you'll have your answer on the above. 
Note if you do not HAVE to aggregate, you may permissively aggregate and do so independently with regard to 401(k) and 401(a).
If you want hasenpfeffer sometimes you have to go down that rabbit hole!

If a 3-year average W2 compensations was established already, you may not need any current compensation for a defined benefit plan. However, you may need a reasonable compensation paid for services, but this is the question to an accountant/tax advisor.

With regard to question 1, the rule is under treas. reg. 1.401(k)-3(e)(2) which says that if you are adding a 401(k) feature to a profit sharing plan for the first time, it can be safe harbor for the first year as long as the 401(k) feature is in effect for at least the last 3 months of the year. For a calendar-year plan, that means the 401(k) feature must be effective no later than October 1st.
https://www.ecfr.gov/current/title-26/part-1/section-1.401(k)-3#p-1.401(k)-3(e)(2)

Does the plan currently permit a discretionary matching contribution, but the employer simply chooses not to make one? If so then the prior year's NHCE ACP is 0% since there were no matching contributions.
If the plan does not currently permit matching contributions, then there is a special rule that deems the prior year NHCE ACP to be 3% in the first year that the plan permits matching contributions. 1.401(m)-2(c)(2)
However, you said the plan is currently using the 3% safe harbor non-elective contribution, so the plan could be exempt from the ACP test, as long as the match satisfies the ACP safe harbor. Depending what you mean by a "small" match it very well might.
Finally, you could just tell the client that if they want to add a match, there is required testing. If the test fails and can't be corrected until after the deadline, because they sent you the data late, then they will be subject to a penalty. It's their decision whether they want to figure out how to get you the data on time, or risk the penalty.

Not explicitly. However the regs do contain a general anti-abuse clause which the IRS could point to in the event of an audit.
https://www.ecfr.gov/current/title-26/part-1/section-1.401(m)-1#p-1.401(m)-1(b)(3)

That is incorrect. The due date depends on a variety of factors, amount particularly, but many plans are on weekly, monthly, or quarterly deposit timing. 
If the tax is not required to be remitted right away, and it is small enough to be sent in with the Form 945, it is subject to the form filing due date, typically January 31 after the year ends. 
The 20% withholding is 945 tax type, so if you look for information on that, you should be able to get additional information. 
Note: When the Form 945 is due is not the same as when the actual $$ must be sent in. The $$ typically have to be sent in sooner. 

There is no 45-day requirement in the law or regulations.
1.401(k)-3(d)(3)(i) requires that the safe harbor notice be provided to participants "within a reasonable period before the beginning of the plan year." Subparagraph (ii) provides a safe harbor that the timing requirement is deemed satisfied if the notice is provided no less than 30 and no more than 90 days before the beginning of the year.
The service provider may have an internal process that they can not update the plan specifications less than 45 days before the end of the year and still comply with the 30 day safe harbor.
Plans using the nonelective contribution to satisfy the ADP safe harbor are no longer required to provide a notice at all, since SECURE 1.0. However a notice is still required if the plan wants to satisfy the ACP safe harbor, even if using a nonelective contribution.

There is no 45-day requirement in the law or regulations.
1.401(k)-3(d)(3)(i) requires that the safe harbor notice be provided to participants "within a reasonable period before the beginning of the plan year." Subparagraph (ii) provides a safe harbor that the timing requirement is deemed satisfied if the notice is provided no less than 30 and no more than 90 days before the beginning of the year.
The service provider may have an internal process that they can not update the plan specifications less than 45 days before the end of the year and still comply with the 30 day safe harbor.
Plans using the nonelective contribution to satisfy the ADP safe harbor are no longer required to provide a notice at all, since SECURE 1.0. However a notice is still required if the plan wants to satisfy the ACP safe harbor, even if using a nonelective contribution.

There is no 45-day requirement in the law or regulations.
1.401(k)-3(d)(3)(i) requires that the safe harbor notice be provided to participants "within a reasonable period before the beginning of the plan year." Subparagraph (ii) provides a safe harbor that the timing requirement is deemed satisfied if the notice is provided no less than 30 and no more than 90 days before the beginning of the year.
The service provider may have an internal process that they can not update the plan specifications less than 45 days before the end of the year and still comply with the 30 day safe harbor.
Plans using the nonelective contribution to satisfy the ADP safe harbor are no longer required to provide a notice at all, since SECURE 1.0. However a notice is still required if the plan wants to satisfy the ACP safe harbor, even if using a nonelective contribution.

There is no 45-day requirement in the law or regulations.
1.401(k)-3(d)(3)(i) requires that the safe harbor notice be provided to participants "within a reasonable period before the beginning of the plan year." Subparagraph (ii) provides a safe harbor that the timing requirement is deemed satisfied if the notice is provided no less than 30 and no more than 90 days before the beginning of the year.
The service provider may have an internal process that they can not update the plan specifications less than 45 days before the end of the year and still comply with the 30 day safe harbor.
Plans using the nonelective contribution to satisfy the ADP safe harbor are no longer required to provide a notice at all, since SECURE 1.0. However a notice is still required if the plan wants to satisfy the ACP safe harbor, even if using a nonelective contribution.

Is this a DB plan, or other plan subject to QJSA? If so, does the plan exempt distributions between $1,000-$7,000 from the QJSA requirements? If it does, then I agree with the other commenters that you can go ahead and distribute the single sum upon plan termination.
If it does not, then you have to make the distribution in a form that preserves the QJSA rights. This probably means buying an annuity contract, or transferring the benefits to the PBGC's missing participants program. 
Optional forms of benefit with respect to distributions less than $7,000 are not a protected benefit under 411(d)(6), so the annuity options could be eliminated without disqualifying the plan. Once the annuity option is eliminated, you could distribute the single sum.
However, if the plan is covered by the PBGC, you may not amend the plan to eliminate a form of benefit after termination, regardless of whether it would be ok under 411(d)(6).

This might be back pay within the meaning of 1.415(c)-2(g)(8). If so, it's considered compensation for the year to which the back pay relates, so 2022 or 2023 in your case.
Does the document address back pay at all?

Paragraph 1.401(a)(4)-11(g)(3)(vi) of the regulations provides conditions for amending a plan to correct a failure of the nondiscriminatory availability of benefits, rights, and features in a prior year.  The paragraph says:
I have never needed to use this to correct an availability failure. It does seem a little strange to me as, if I'm reading it correctly, it only says to correct the failure effective prospectively and keep that amendment in place until the end of the next year. It doesn't seem to do anything to relieve the participants to whom the benefit, right or feature was unavailable during the prior year.
Would be curious to know if anyone has actually relied on this method in practice.

...which would be a significant problem in a DB plan, as opposed to an IRA or DC plan. Based on the 2025 limits, the 415 maximum lump sum is about $3.5 million. If the husband and wife are the only participants and the assets grew to $5 billion, then they would be stuck with $4.993 billion that could not be distributed, but would be a taxable reversion to the employer, plus subject to $2.5 billion in excise tax.

It's not straightforward to convert from the table to a factor. You really have to use software to do it. I posted a spreadsheet here a couple of years ago that will calculate the factors for you, if you paste in the table and enter an interest rate. Let me see if I can dig it up again.
 
Edit: here it is
 

Is this a DC plan or a DB plan?
Also, he was working for 5 days after he died???