The totality of your injury argues for an HSA or equivalent instructor as they will work with your abilities, not your disabilities to make diving possible. We have participated in training classes with Traumatic Brain Injuries (TBI is the signature wound of Iraq) to SCUBA Dive. This is challenging as the memory is affected, but we teach, teach again, wait two weeks and evaluate and reteach if necessary.
I would suggest you start, with doctors permission on what exercise you can do now to keep your strength up and build more. Any type of aerobic exercise you can do would be terrific, bicycle, walk as fast as possible etc Build up slowly. Swimming is a very good total body exercise. Work on the old melon too. Exercise the brain to improve memory and keep the mind working. Crossword puzzles, sudko (or what ever that numbers crossword type thing is called).
From your most recent comments, a refresher (aka total recertification) may be in order, but the instructor will decide that when they talk to you.
Good luck. I wish I was in your neck of the wood so I could help, but you have local people there that have posted here.