Belgarath

A safe harbor match can not require hours in order to receive it on a year-by-year basis, but it can have a service requirement for initial eligibility.
No, you can have different eligibility for deferrals and safe harbor match. The major consequence of this design is the loss of the top heavy exemption, as Bri noted earlier. Under this design you are technically doing an ADP test for the disaggregated portion of the plan covering otherwise excludable employees, since that group is not covered by the safe harbor match. It is unlikely that there would be any otherwise excludable HCEs, so that group should always pass the test automatically. But it's something to be aware of.
No, you can have a service (hours or elapsed time) for initial eligibility for matching contributions, including safe harbor matching contributions. If your document uses a checkbox-style adoption agreement, there are probably options for this.
A plan that consists solely of deferrals and matching contributions which satisfy the ADP and ACP safe harbors is exempt from top heavy. This is determined based on the contributions that are actually made to the plan on a year-by-year basis. A plan can permit non-elective contributions but will not lose its top heavy exemption unless non-elective contributions are actually made (or forfeitures allocated) in a given year. Likewise, making non-safe harbor matching contributions will also cause the plan to lose its top heavy exemption.

Is everyone who is eligible to defer (regardless of whether or not they elected to) also eligible to receive the safe harbor match? In other words, is the eligibility the same for deferrals and safe harbor match? Or do they have (for example) immediate eligibility for deferrals, but 1 year of service for match?
If everyone who could defer would be eligible to receive the match if they deferred, then you are good on the top heavy exemption.
However if there is anyone who is eligible to defer but not covered by the match then the plan is not exempt from top heavy, even if no contributions other than deferrals and safe harbor match are actually made.
However however if the only employees who are eligible to defer but not receive a match are long-term part-time employees (who are eligible solely because they met the LTPT eligibility criteria) then the plan retains its safe harbor top heavy exemption.

Probably not a CG. But remember, even though there is no family attribution from siblings, there could be, for example, certain options to purchase some or all of the other 21%, that would count as "ownership" for these purposes. Most CG determinations frankly don't seem to take into account such intricacies, which is why we always advise clients to seek legal/tax counsel before making the determination. Or at least do a deeper dive into the rules.
P.S. I just read another post of yours where Cuse makes the exact same point with regard to options.

If you use a service provider’s materials, apply your own sense to a classification of a provision as mandatory or optional.
Some service providers classify a SECURE 2019 or SECURE 2022 tax law change as “mandatory” for the business to provide services most customers likely call for, or that are essential to how the provider’s systems operate.
For example, about the tax law change that permits a plan to provide for a § 401(a)(9) required beginning date an applicable age of 73 or 75, at least one big recordkeeper classified this as mandatory. But a plan may provide, without failing to meet § 401(a)(9), an applicable age of 72, 71, 70½, 70, or even younger (if it doesn’t provide an involuntary distribution before normal retirement age). So, I might classify the change to 73/75 as optional. By contrast, a provision about whether some beneficiaries must complete a distribution within ten years from the participant’s death might, depending on the plan’s other provisions, be needed to tax-qualify.
Also, some of the service providers’ outlines don’t show completely or conspicuously an exception or variation for a governmental plan.
If your work is about a governmental plan, consider that the State’s laws might restrain which provisions must, may, or must not be included or omitted. Those laws might include an enabling statute that grants, and limits, powers to establish and maintain a § 403(b) or other retirement plan. And it might include laws about collective bargaining or discussion with associations of employees and retirees. Those laws might make required or obligated a provision that under Federal tax law is optional.
Further, consider that some provisions might be stated by an annuity contract or a custodial account, rather than in “the” plan document.

Keep in mind that plan documents have default beneficiary provisions and the Plan Administrator cannot force a participant to make an affirmative elective.  Also keep in mind that a terminated participant who has an accrued benefit can change their beneficiary designation while they remain terminated.
It is good practice to encourage participants to make affirmative elections, and to periodically remind participants to review their affirmative elections.  Some plans have generic communications that center around how life events - including rehire - that can impact benefits and those communications include the reminder to update beneficiaries.

The title asks "must?"  No, of course not.  However, the employer's procedures for rehires probably do not differ from its procedures for new hires, so Yes.  But more to the point, it would be enormously foolish to assume nothing has changed in this person's personal life "after several years".

Lincoln Financial has a good one on their Secure 2.0 website. https://www.lincolnfinancial.com/public/general/secureact 
It is labeled reference guide for DC Plans.  
 

I think @jsample is referring to an "unallocated suspense account" rather than a forfeiture account.  I don't recall where this is discussed, but I don't think its EPCRS.  If I remember it correctly, the excess is moved to an unallocated suspense account and is not an annual addition.  You then have to use the assets in the unallocated suspense account before you make any further contributions, and they are an annual addition when allocated.  So from a deduction perspective, you can deduct it when allocated, but not when deposited.

This is all wonderful news.  I think probably 80% of my clients (i.e., the smaller ones) will not adopt this provision.  Wonderful sentiment, but execution is so incredibly infeasible.  And if you have less than 50 employees, you might have one person every 3 or 4 years eligible for this, and how many of them can actually contribute more than $30,000???  I just don't get it.  This is wonderful for IBM and Microsoft and Amazon, but down here in the weeds it's the last thing anyone wants to deal with.

One item that occurs is that the in-plan Roth rollover would normally require a distribution fee to be charged to the participant's account. That said, I'm not sure most employers will be willing to go through the hassle with payroll/payroll providers, reporting, etc. Our clients already have enough trouble just properly allocating pre-tax and Roth deferrals to the correct account - I can only imagine the screw-ups if they attempted employer contributions as Roth.
I'm not a fan of the concept.

You don't want free advice.  Someone needs an ERISA attorney that is good with ESOPs.  I am serious here.  I have been working on ESOPs since the mid 90s and I wouldn't dare advice the client how to clean this up. 

At least some pre-approved documents provide for an "other" election for excluding participants from the Safe Harbor contribution, where they specify that it must be "an HCE, or" ............... so I don't see any prohibition about specifically naming an HCE as excluded. But I'll ask this - why? Given that documents can provide complete flexibility to exclude all HCE's, but make a "discretionary" Safe Harbor to "any or all" HCE's - what would be the point of limiting the flexibility by specifically naming one HCE?

Yes it meets the the requirement by making a single entry date on the first day of the Plan year and no one is kept out more than 18 months which is acceptable.
Yes folks hired in June have the shortest 6+ month eligibility, May 7+ months, April 8+,....,Dec 12+ months, ... July 17+months
 

I would defer to the company's accountant, but if the consolidated company files a consolidated tax return, then any or all companies within the control group can contribute whatever amounts. However, if A & B file separate tax returns I believe each must contribute and deduct their respective amounts. At least that is my recollection, but again, a qualified tax accountant for the company(ies) should be able to answer definitively.

One of today's items in the Benefits Link newsletter had a write-up on hardship distribution self-certification. The following is an excerpt. I don't read Section 312 of SECURE 2.0 as containing any such restriction. What am I missing, if anything?
Employers may now rely on an employee self-certification that they have experienced a hardship and that the employee has no other funds available to satisfy the hardship. Self-certification is only available for the first hardship request during a plan year.  If the participant requests more than two hardship distributions in one year then the employer is required to have physical proof of the hardship. 

You don't know how hard it is to resist ......

Probably not a CG. But remember, even though there is no family attribution from siblings, there could be, for example, certain options to purchase some or all of the other 21%, that would count as "ownership" for these purposes. Most CG determinations frankly don't seem to take into account such intricacies, which is why we always advise clients to seek legal/tax counsel before making the determination. Or at least do a deeper dive into the rules.
P.S. I just read another post of yours where Cuse makes the exact same point with regard to options.

Probably not a CG. But remember, even though there is no family attribution from siblings, there could be, for example, certain options to purchase some or all of the other 21%, that would count as "ownership" for these purposes. Most CG determinations frankly don't seem to take into account such intricacies, which is why we always advise clients to seek legal/tax counsel before making the determination. Or at least do a deeper dive into the rules.
P.S. I just read another post of yours where Cuse makes the exact same point with regard to options.

All great advice. I'll also mention ERISApedia. A great resource, with some outstanding bells and whistles available.
Furthermore, the knowledge and generosity of their time and expertise among the many participants on this board has helped me immeasurably over the years. While I'm at it, yet another thanks to Dave and Lois Baker for doing such a great job in providing this forum!

I think the misunderstanding that many people have is that if the participant did not fill out a beneficiary form/designation, then the account is subject to the terms of a will, or if no will, then intestate rules. 
It isn't. 401(k) plans have default beneficiaries written into the governing plan documents, so that in the event a participant passes without a affirmative beneficiary designation, there is a default beneficiary. 
Typically that is something like spouse, children, estate, but it varies. Read the plan's document carefully.
Even if the estate is where the benefits are to go - they go there because of the beneficiary rules in the plan document, not because of the application of a will or intestate laws. So If everyone else pre-deceases the participant (not what we have in this post) the estate is the named default beneficiary under the terms of the plan, and gets the $$ because of that. 

I look at this one step at a time.   When uncle dies, plan assets go either per a beneficiary designation *or* if none, per the terms of the plan.  I would guess that the spouse (aunt) is the bene under the terms of the plan - so those assets go to her - whether she exercise control over them or not.  Uncles will is irrelevant.  Only a valid beneficiary designation or the terms of the plan govern.  So, when aunt died, assets go per her bene designation (if any) or per the terms of the plan - and uncle, uncle's estate, and uncles trust have no bearing on aunt's distribution of her interest in the plan.  Aunt's representative (estate) or others would be entitled to those benefits - absent some fact not disclosed.  The court has NO JURISDICTION over the plan assets until paid, and cannot direct those assets to be paid to the trust, and whether it is a pass-through is really irrelevant..

All great advice. I'll also mention ERISApedia. A great resource, with some outstanding bells and whistles available.
Furthermore, the knowledge and generosity of their time and expertise among the many participants on this board has helped me immeasurably over the years. While I'm at it, yet another thanks to Dave and Lois Baker for doing such a great job in providing this forum!

Also, be careful of this. If dad has any exercisable option on stock owned by son he is deemed to own that as well.
(e)Constructive ownership
(1)Options If any person has an option to acquire stock, such stock shall be considered as owned by such person. For purposes of this paragraph, an option to acquire such an option, and each one of a series of such options, shall be considered as an option to acquire such stock.

Agreed! And even if you went to a VCP filing on this (which would be an absurd waste of time and money IMHO) I'm very dubious that the IRS would approve it.
 

I think the latest required restatement will be in 2076.