Hi, Jerry -
I wish I had a direct answer to all three of your questions, but I don't. However, focusing on your question about corporate TESL for employees, although I don't know of any companies that do that (there may be some), I do know that there are companies that hire people to come in and teach business writing classes, and I'm talking about very basic stuff -- grammar, punctuation, etc. This work is usually done on a consulting basis, and although there's competition out there from companies like Fred Pryor, National Seminars and the like, I believe companies would appreciate getting the training from a specialist in the field. It's worth looking into.
Your age shouldn't be an issue (and you shouldn't make it one when approaching prospective customers / employers).