Note: last I heard, short term rentals in our county are only permissible in primary residences.

In this case, you might want to check county zoning to see if it's permissible (Texas is just about the most landlord friendly place on earth). 

If you cannot easily change the land designation, you can always appeal to the municipal government (usually a council) to give you special permission to overcome the obstacle(s) that you have for dividing your land as you would like.This is all easiest if you are working in the same direction as the council/planners.

- I live in a state where self-help eviction is permissable, so long as it's written and agreed to in the signed lease. 

I usually don't ask for permission.  

Also, if it is rural, you still have to get permission from the Neches River Authority as well as the state for septic if it's in Livingston.

You may need to speak with a securities attorney, as opposed to a real estate attorney though to figure out what is permissible. 

You have clearly done the math, so if it is permission you need then I say go for it.

He's been making money off your property without your permission.

In Ohio, we don't have to get permission to enter we only have to give notice for legitimate purposes like repairs or pictures or appraisals; check the laws for your state and if possible get inside and get pictures.