Call a lawyer and have them rattle his chains.Have him threatened with a lawsuit for failing to remove you from financial responsibility for the home you no longer own.

Does anyone in the St Petersburg, Florida area have any experience with addressing an ADU/Guest Suite/Mother-In-Law suite?

If you do not want to list an address where you can be reached, then you'll have to pay somebody else (or beg a friend) to use their address and they are then authorized to be be served with a lawsuit on your behalf.For marketing purposes, a virtual physical address is appropriate.

Essentially all that means is now you have 6mm for a lawsuit someone may file against you vs the initial million.

Moreover, with a mortgage, you are not as much of a lawsuit magnet as you are with a paid-in-full property.  

One of these strategies is to purchase a SF with a mother-in-law suite or some kind of addition and rent out both sides, allowing for similar income to a duplex.

Or alternatively, you could consider using a 5% down conventional loan next year to purchase a single family with an ADU/in-law suite that you could rent out.

(claimed she fell) However, the lawsuit went nowhere since it was baseless.  

Before spending a few grand on a lawsuit to collect the unpaid rent, I would first determine if its worth going after them. if this is a C rental then its not worth it as you probably will never collect compared to if its an A area.

There are countless guest lawsuits that will wipe out your whole livelihood, property portfolios and financial soundness.