1. The loss of micro-vascularization in the bone to radiotherapy is permanent, but for brief periods of time, the lack of healing response because of this (important for a vaiety of situations including implant placement) can be overcome with 02 dives in a chamber. This doesn't make new microcapillaries appear, it makes whatever you have left more efficient.
2. If you have ORN you will know. You will have a non healing wound in your mouth.
3. I think there are several people here who can comment on this who have actually been down this path, but there isn't much of a down side to the dives in comparisom to the benefits.
4. It is unlikely that 02 dives are going to help with your neck stiffness.