Wow, what a great build!! It looks like you have been doing this forever even it you haven't.
It looks like you have been very through on thinking things out and probably have this covered but if not I'd look into it. I think I'm going to have to move my cage up about 1 1/2 inches to get helmet and roll bar padding room that I need. Helmets have grown since I first started and the padding on the tubes takes up more room than I first accounted for due to the curved shape. You most likely have already checked but if not put the cage padding in, the helmet and suit on along with the belts and make sure you fit and still have forward vision.
The other problem with these lay-down cars is the steering and all the controls need to be very specific to anyone that will drive the car. I have my foot pedals and steering both setup so they can be moved between drivers if that happens.
I'll be interested in seeing how the hand brake works. I've wanted to go that route but there again laying down in there it is hard to move your arm in the direction you need to operate one. I'm having the same problem with the gearshift as I hope to stay away from an air shifter. I'm looking at having the shifter swing just under the steering in an arc parallel to the bottom of the car with the pivot point forward on my right elbow, if that makes sense.
With the limited arm movement with restraints and just how your arm can bend, getting the hand brake, hand shifter and the fire bottle controls and the chute releases where they all can be worked can be a challenge. Much easier in a car where you aren't so prone. I wish I would of worked on this area earlier in the build.
What are you planning on for power-plants?
Keep posting as you have some really good ideas,
Sum