When I did my early high level due diligence it looked to me like they weren't issuing any new permits, but I would love to be wrong on that.I do know, having just stayed there in an airbnb, that even for existing permits the rules are extremely strict. 

So, if they have a lease with 8months remaining, and you buy on an owner-occupant mortgage requiring residency within 3 months (typically) you would have to take the vacant unit and honor the remaining term of the existing lease.When that lease expires, I am guessing, you have every right to not renew their lease.  

In Florida, coastal areas tend to offer the best opportunities for STRs, especially given the saturation in the theme park areas of Central Florida but I'll let the full time operators chime in.

We’re developing onto an existing residential property through the addition of an ADU we plan to rent medium term.

I believe we’d also need to cross-collateralize our 2 existing properties (Florida and Virginia) as part of this.

The size of the deal makes a difference.Larger apartment complexes are typically not advertised outside of the Brokerage webist and lists, so unless you're on their list, you may not know those deals exist. 

If a lien already exists, but hasn't aged, to where it can be converted, then it's worthless?

While it’s true that most agents gauge a potential client’s seriousness by the existence or absence of a pre-approval letter, I think you can ease concerns by being honest with agents and voicing a preference to receive a lender recommendation from a trusted partner - which is what your agent should be from beginning to close.

We are looking for $3.6million which would take out the existing mortgage on the property of $535k.   

There's likely not a single lender that exists that will allow a 100% CLTV at purchase.