So Jakyasar, you're talking here about a non-401(k) plan (no employee deferrals), and presumably the employer was on extension for its return, so they/you are using the ability under SECURE Act change to adopt a plan retroactively, right? I assume a noninstitutional trustee (e.g., the company owners), because most likely a bank or other institution would not have allowed opening the account in name of trust without document.
I would say: (1) To some extent the issues posed are novel, since the SECURE Act change is new.
(2) Once they signed the plan documents, you certainly have a trust and I think you'd potentially run into more trouble if the trustee tried to take the money out now, and then put it back in.
(3) I guess the IRS could argue that the trustee owes taxes for the period from 5/23/2022 to 6/3/2022 (a little over a week), since the money was not in a qualified trust. But as long as the total allocations for 2021 don't exceed the 415 limit, don't see how you would have a qualification issue.
(4) In the small corporation setting, when issues like this have been dealt with on audit or determination letter process, sometimes lawyers will argue successfully to the IRS that since the individual who made the deposit was a 100% shareholder, or if all the directors were aware of and had approved the deposit in some way, the plan had been adopted. In theory the success of this approach depends on what the plan documents say. If you're lucky, (a) the plan document won't say that it is not adopted until signed, but rather will just say it needs to be "adopted," and (b) the corporate resolution that was provided will authorize the officers to "sign such documents as are deemed appropriate to carry out the purposes of these resolutions, etc."
(5) You could always put a memo in the file saying that you had talked to the employer and the appropriate body (board, sole proprietor/boss, partners) had considered the plan's adoption and decided to do it before the deposit was made, assuming those are the facts. Not saying that would be bullet-proof if this was $1 million and someone had it in for this employer (e.g., the business is about to go bankrupt and the employer is trying to put money our of a creditor's reach), but it could help, e.g. in an exam. The issue could certainly be spotted in an exam, because the IRS always checks dates of documents. But absent unusual circumstances this does not seem like the sort of thing an agent should be upset about, especially given that the SECURE Act provision is so new.
Sure, would have been better to sign the documents first.