Hi JoeyD,
Before i was passing I'd use the restroom according to the way I dressed. To know for sure that others realize based on you dressing your attempted gender can be trickey unless you are wearing a dress for instance. The shoes are in my opinion very important as they can be seen not only while walking but also while sitting on the toallete. I,m assuming you'd never stand and pee!!!!
So just be quick, but don't avoid looks that might meet you in there. Give a smile as well! The big difference between the guys and ladies restroom is the guys avoid looks, seek no interection and the ladies do the opposite.
If you get in trouble with the manager again stick to your original story and bring up the loyer if he is not willing to listen. Usually people react to the threat of the law suit if they have ignored you until then.
If you run into trouble again and get kicked out, bring your doctors letter to the manager and ask your name to be cleared. If they call the cops, they usually listen better to what you have to say than the theater managers.
Good luck!