My only concern is I believe most ADUs all tie into the same utilities and would be 1 bill.

If you aren’t saving the money up to buy another property chances are nothing will pay a higher rate than paying it towards your mortgage.

I chose to invest most of those gains into a trucking company due to the lack of patience at the time and my 10 year experience being a tractor trailer w-2 driver.

But since he sold the house, the entire insurance payout ends up taxable now. 

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

The stock market is up something like 80% of the days long term.

It may be your most profitable asset. 

If this is going to be a investment home you will have a hard time getting a cash out refinance with most lenders as an invesment mobile home.

It also confirms the amount of security deposit and the lease ending date.

However, this has to be the primary residence of everyone on title in order to qualify and even if that were all true, we're still only up to $1M that way.I agree with the other commenters, the parents should continue to own it and live in it as their primary, leave it to the daughter when they pass, and she inherits it at the stepped up cost basis.