I really don’t want to lose this buyer—what’s the best way to make this work?

Quote from @Brandon Morgan: If you screw up a deal with a friend or relative, you will lose money and the relationship.

I attempted to provide the most transparent and up to date information on this site. 

Meaning if the borrower stops making their payments the investor can lose all of their investment.

I would start there and think of anything that I could do to stay in the house for a while as it sounds like you would be losing a bit of money with the other options. 

But consider this: if your time is worth $100/hour and you spend 40 hours a year on tasks you could outsource for $25/hour, you’re effectively losing $3,000 of potential value each year. 

It's not a booking site.

You don’t want to add it to the rent and pay it yourself because you’ll lose the “Tenant compares the rents game.”

Losing control of the process matters. 

I dont know the delivery time on a ADU but I'd assume same with site plans and zoning. but how do you get your money out of that?