You are correct on the issue of teeth. Regardless of what can be done prior to radiotherapy, the jaw bone (Mandible) can be damaged to where it may literally die. And when that happens the teeth roots cannot keep the teeth alive and strong. Dentists will recommend fluoride treatments however from what I read, fluoride is not good for bones.
I had a mandible section replaced with part of my fibula and blood supply flap to keep the bone alive and allow it to heal with the remaining live sections of mandible. I was 65 when this was done at Mayo Clinic and it has worked well. I can eat easily now. Have you considered something like this as an option?