Lou S.

I don't think unlawful is the right word.  You won't get arrested for this... just lose tax status. 

IRC 401(k)(2)(B)
Also, paragraph 1.401(k)-1(d) of the regulations.

If they are employees, they count.

100% correct. 
Executor - you need to listen to David. 
it is a very common mistake that people think their pre-nup has any bearing on the 401(k). It doesn't. The spouse has to sign the beneficiary form waiving their benefit if the participant wants any portion to go a non-spouse. The spouse has to sign after the marriage occurs. Some basic searching online for court cases will demonstrate this. 
The plan administrator doesn't care what the pre-nup says. All they can follow and should follow is the terms of the plan and a valid beneficiary form. 
If the widow received the 401(k) $$ and some other agreement says they shouldn't, well then the estate or whoever typically would take legal action to try to resolve that, against the widow. It isn't an issue for the plan. 
Any competent estate lawyer would know about this when drafting the pre-nup and explain it to the parties. And that is why a post-marriage checklist exists for a reason. But no one can force people to sign anything. I suggest you contact an experienced family law attorney if you want to pursue it further. 

Pre-nup is not relevant.  The Plan must follow its own rules for distribution and who is defined to be the beneficiary.  Hint: likely, the plan defines beneficiary as "spouse".

ACP testing is required for after tax contributions, safe harbor does not exempt you from this.

I believe the answer to the original question is yes, assuming that the annuity purchase was handled properly. See SECURE 2.0 sec. 204, section 1.401(a)(9)-5(a)(5)(iv) of the 2024 final regulation, and 1.401(a)(9)-5(a)(5)(v) of the 2024 proposed regulations.

You need to contact your employer/former employer. 

Wrong on 5500 or 5558?
 
Either way, I think you’re okay as long as the EIN and PN are correct. 

We typically don't see this if the plan has been terminated for a while.  Usually when amounts do appear well after the termination and final reporting of the plan, the amounts are attributable to a settlement of litigation.  It would be absurd to resurrect the plan, update for recent legislation, pass around some pennies, make payments, amend the prior final 5500, prepare a few more 5500s for the intervening years and file another final 5500, send SAR to participants, ...
Let's get real and not overthink it.  The trustees or plan sponsor should ask the brokerage firm to close the account and write off the amount.  If the brokerage firm adamantly refuses, then one of the former service providers likely will be willing to send an invoice to the brokerage firm to close out the account.

She is subject to the same eligibility conditions as all other employees.
Now if you are asking if her unpaid service counts towards the 1000 requirement, that's not an argument I'd like to have with the IRS on audit.

She is subject to the same eligibility conditions as all other employees.
Now if you are asking if her unpaid service counts towards the 1000 requirement, that's not an argument I'd like to have with the IRS on audit.

She is subject to the same eligibility conditions as all other employees.
Now if you are asking if her unpaid service counts towards the 1000 requirement, that's not an argument I'd like to have with the IRS on audit.

She is subject to the same eligibility conditions as all other employees.
Now if you are asking if her unpaid service counts towards the 1000 requirement, that's not an argument I'd like to have with the IRS on audit.

If it's not an annual addition because of their 415 limit being zero, I'd argue those aren't actual deferrals triggering the THM.  (As opposed to an ADP refund)

Sounds like your "company" has a controlled group issue or a coverage issue.  Maybe the owners created a DB plan to fund a very high benefit for themselves (10%/year of service - i.e. 100% of pay lifetime benefit after 10 years) in 2020 and is now realizing the plan didn't cover a sufficient number of employees to be in compliance with the applicable laws.  This notice is essentially telling you that although you might not have known you were a participant, you might be, and since you might be a participant, we need to tell you that we are freezing the plan until we figure this out.
Sounds like a mess for your "company".  From your perspective, you may end up getting a benefit you never knew you were entitled to, or you might not.  
Since they sent you the 204(h) Notice, I suggest that you ask for a copy of the plan's SPD.  That will describe how the plan works and what the benefits are.  You could also look at the form 5500 which is available in public domain. It will also contain an abbreviated plan description  Easiest way is to search your company's EIN, but plan name also works.  https://www.efast.dol.gov/5500Search/

The 415(b) dollar limit is adjusted for benefit commencement dates later than age 65 using the 417(e) applicable mortality table and 5% interest.
Note that the 100% of compensation limit is not adjusted. So by 85 (much earlier than that, in fact) the compensation limit will be lower than the adjusted dollar limit and will control.

Above is correct

The plans may be optionally (the word in the reg is "permissively") aggregated for coverage and nondiscrimination. If they pass separately, then they do not need to pass combined. If the plans are aggregated for either coverage or nondiscrimination, they must be aggregated for both. In other words, you have to use the same options for both coverage and nondiscrimination.
All plans of the employer must be aggregated for the average benefits percentage test.
If both plans cover a Key employee, then they are part of a required aggregation group for top heavy purposes.

I didn't follow the link but I think that New Jersey thinks that if you deferred income while a NJ resident, you are supposed to pay tax to NJ on the income no matter where it is received. That is widely - universally - ignored.

You can’t merge a 401k into a 403b. 
But I think that’s the least of the issues. 
seems like the guy is wanting to use a not for profit entity to pay the wages of the people that actually work for his for-profit business. 
I would have to imagine there are legal issues with that and I would avoid it. If you can insulate yourself by having his CPA and an ERISA atty drive the decisions, then you should be good. 

Yes

Yes, there is no control group, but if each entity had the same type of plan then the 100%/51% owner would have an aggregated 415 limit.

A brother-sister controlled group requires 80% or more common control. It doesn't sound like that exists here.

Let them know you'll be buying an annuity with a QJSA unless they can get you a finalized QDRO? They can't hold the Plan termination hostage by saying we don't have a QDRO 2 years on.