Look at putting an airbnb in an inlaw suite or something like that.

I'd also open up your options to homes with in-law suites that you can section off into another unit.

First rule of suing, sue everyone so you will definingly be named in the suit especially since you know about it. 

If this happened in Indiana, I'd pursue a small claims suit because they are quicker and there's less hassle.

I'm trying to get creative with how to add an ADU / Mother-In-Law suite.

I don't believe you can just go back to an addendum that suits your needs.

It is important NOT to commit to anything except to continue to explore an arrangement that suits you both. 

We have also been looking at single family homes with finished basements or in law suites/guest suites.

Those are usuallyPeople with problems not suited for normal market sale and financingInvestors who understand the investment aspect of owner finance and are open to that.I find the 2nd group to be the most target-able. 

So that question would be best suited for a local lender that can guide you through what they would need to see income wise as well as what you qualify for.