You definitely want to use a thorough screening method like rentprep.com that gets their income, credit, background check, and call at least 3 references (make sure to verify them by address/job and then their name).  

What's the highest success percentage method to find a previous tenant's new address for this? 

If you are open to non-traditional methods of financing, you could use a debt service loan to bypass that requirement.

Offer Geek Homes LLC, OGeek LLC, Zecharian R.

I can't tell the future, I just will tell you markets behave when supply and demand are close(r) to be met.

We've also advised this method to our investor clients that borrow from us.

I want this to be acceptable long term if I keep this method of financing. 

Definitely do more research on the BRRRR method as that sounds like what you're trying to do.

As of now that definitive list is ADUs are allowed in the following zoning districts: R-1, R-2, R-4, R-6, R-10, RX, OX, NX, DX, and CX.

This is the simplest method, but it also means your tenants are more likely to abuse the utilities by leaving windows open with the heat or A/C running, leaving lights on, ignoring the toilet that constantly flushes on its own, etc.3.