I want to make sure I comply with the law 100%.

Serious buyers will comply - - see my profile -- this is how I handled my sale.

So you want to screen for a good tenant/buyer and not just accept the first party who is interested but has little chance of ever qualifying for a loan and cashing out the owner.Also I strongly encourage you to consult an experienced real estate attorney to make sure you are complying with state laws (specifically your state's real estate commission laws). 

It's three years old and has been revised multiple times since then to comply with current laws.I use Vesper and Stallings when I need addenda written, and I'm sure they already have a current lease they could sell to you.

No one should buy here without truly understanding the implications as the fine is $1500 per month until complied with,The law takes effect Sept 15 but I have it on good authority the clock will be delayed until Jan 1.

If you get pushback on the fence, as for previously existing written permission - - give 60-day notice to comply or terminate.

It makes me wonder if they have already tried and have been informed that they are not eligible, or rejected mitigation offers or didn't comply with the terms.

You've got a LOT on your plate and only you can judge the family dynamic that you'll be working within but IMO you want someone you can dictate to and expect them to fully comply; if they don't they get replaced.  

If anything you should feel good that the platform is complying with the rules, hopefully that also carries to their underwriting :).

Not complying with the terms of the lease, any lease, is terms for termination, ask any attorney.