I would never lease any of my horses, ever. Unless it's my trainer I refuse to ever let anyone ride my horses. That's how protective I am of them.
In your situation, because of the fact you are currently carrying another human being (kudos to you for actually taking time off riding for it)I think it's great idea especially since you have been trying to sell the horse.
The rules you have laid out are very good. You should talk to the barn manager, or your trainer about putting a contract together. They probably have one already made you can use.