In good faith, I began reaching out to various individuals, only to discover that the offer had yet to be officially accepted; they were still deliberating.

The first thing I would do is issue an official notice stating how much they owe as well as a lease termination and if they don't pay it within the period prescribed by your state and leave they are being evicted and or sued in small claims for the past due rent. 

If you know how to search the public records-clerk of court, official records--you'll find recorded liens.

How can I make this an official three unit house. 

There is not an “official” guide from HUD, but BiggerPockets has a great article here on the differences:https://www.biggerpockets.com/blog/real-estate-property-clas...

I bet the lobby on that has already started.  

On a primary residence, the child must live in it along with stating officially that they want the exemption within 3 mo I believe.

Perhaps if STRs develop into more of an official asset class in the yes of the secondary capital markets this could become a thing, but this is likely years away if it occurs.

Yes my team and I have met this week to review a newsletter roadmap and are excited to officially send out our first.

Indeed, I can consult with an attorney for "official" advice, but I'm curious if anyone else encountered such an issue.If it was my fault for missing the date, there would have been consequences (e.g., keep my down payment and exit deal)What consequence can they face for this delay?