Nitrous plumbing A1A...??..

That sound like the ticket for the wire path.

My one thought is (if it were my car or job) i would think about not daisy chaining the power to the retard box. I would probably want the to have its own hot or at least the hot that feed the ignition?

I would not trust all those parts and connections in front of such an important part. If the retard fails maybe bad things?

My thought is just build the system with worst case scenario planned in.

You are as always, doing a fantastic job.