Principal to be returned in full at the end of the three-year term. 

Most of the points you're making are making relate to that.Yet, your post was about a short-term rental or about an investment property.

No permit required for long term rentals in un incorporated Cobb.

Turnkey buy and hold with long term renters is the best way to get your foot in the door with RE investing.

Does anyone know of a software to calculate medium term rentals?

If you have a lease that is currently in place, you have to keep the terms the same until it ends and then have them sign a new lease with the new terms (give them the proper notice required).

The solution going forward is don’t agree to terms with wholesalers that have study periods unless the DD period is reasonable and/or you can still market until the $ goes hard. 

I’ve got a single family townhouse long term rental.

Short pertinent bio: I am one of those guys who can do just about anything I put my hand to.

Just remember, life's too short to own C class properties!