One more thing I can offer to all the great advice that you've gotten so far is a cheaper alternative to a therabite. During your next visit, ask the Doctor or nurse for a couple of dozen tongue depressors. Take them home and stack up as many as you can fit into your open mouth without straining unduly. There should be a bit of strain to get them between your teeth; just not too much. Then secure them together with an elastic band at one end. Slip them between your teeth several times a day, and levae them there for as long as you can each time. It'll probably only be for a few minutes, but if you do it multiple times a day, you'll keep your jaw muscles loosened. In between uses, drop the ends you had in your mouth in a very dilute peroxide and water solution to keep them as germ free as possible.
They do effectively the same thing as a therabite, without the cost, and are very easy to come by.
Lastly, be prepared for the effects to hit early. I only got 5 days of rad in before I lost my sense of taste, and had burns in my mouth and on my neck within the first 10 days. The following few months were really tough, but I managed to get through them without a PEG. Remember to keep swallowing, every day. I think taking your water orally is a really great idea, since it maintains muscle memory in your throat. Don't try and tough out the pain; you will be really sick, really quickly if you stop taking enough nutrition and fluids because it hurts too much. I wound up in hospital a couple of times, and literally overnight I went from feeling O.K. to being really, really ill.
Much luck...it'll be done, and you'll be on the road to recovery
Wayne