Hi Cedric,
An interesting posting.
I have a few questions.
If you have all these backup facilities, why would you be diving solo in the first place?
Many of your suggestions are practical and well considered, but if you are going to have someone on standby to do a rescue attempt, would it not be more practical to just dive together in the first place. I dive solo quite frequently, though seldom on rebreather, and often because conditions are right and I need to go do the survey, and no-one is available on short notice that I am willing to dive with, so I let someone know when I expect to be out of the water and make a phone call to confirm I am out. I always assume if I get into trouble I must get myself out and plan accordingly.
Cheers,
Peter