Some thoughts on the boat design etc.
Put in a permenant ladder on the transom down to water level. The stories of everyone jumping off the boat and then finding the ladder was not down so no way to get back onboard still give me a chill down my neck.
To dive safely from a sail boat you need at least 2 people on deck not 1 at all times. The location of the helm means that the person stearing cannot see the water anywhere near the front of the boat so it is really easy to run over a diver.
A better solution is to dive from a rubber duck and just use the sailboat as a base however that does leave the nondiver sitting in a small inflatable while everyone else is having fun diving.
Air supply for the compressor. If the compressor is going to be permenantly located in the engine compartment how can you insure that the input air is not contaminated by exhaust or other fumes. Remember that there is vibration, a damp salt atmosphere, rust etc. Its going to be hard on any trunking you install.
Where are you going to run the whip's to, this needs carefull thought as the tanks are heavy and you don't want them rolling around while they are being filled.
I would not bother with nitrox, I think this would be far to much trouble on a sailboat.