My STR is in a city where it is legal in private but not in public.

Once the note is accelerated the seller/buyer either needs to pay off the entire balance within a legally prescribed period of time, renegotiate a satisfactory solution with said lender, or face possible foreclosure filing. 4.

One day the SEC my let them know the fine when they investigate a deal that has a complaint of some sort. 

Different banks will have different lending guidelines, legal lending limits, fees etc.

Kiavi, the only lender who'd touch a land trust, ran it up the ladder with their legal team.  

Since the lease was with both then you need to make the check out to both, to do otherwise would be making a legal determination which is what the domestic court is for, and where they can take their issue to resolve. 

Is there any legal guidance suggesting I can or cannot do this?

Legally speaking, if you have agreed to an installment sale, I'm not sure if the stepped-up basis is applicable.

At which point am I legally allowed to relist the property for rent again?

You will find many recent posts from investors who have lost all of their syndication investment, and then talk about bringing legal action against the syndicator.