@RS485 made a big breakthrough and I was able to replicate it on my actual units. We now have the system working as designed. It is really a cool idea and concept. I am finishing a video which I will post tomorrow (and it will be a bit shorter too) explaining the piece of the puzzle we were missing. No spoilers, but the sync plugs are the same on each unit, but that is just so they are interchangable, the connections are the key (and pretty brilliant).
This system was very short lived because it worked great with the original "shelf electronics" parts style boards that had actual timing circuits on the board made of 555 chips that you could "tap". Once proprietary chips came along just tapping into the timing circuit of the supply wasn't as easy as picking a spot to insert inputs and outputs, so this specific tech died with this type of board.
Regardless, I am editing the follow up video and will post it when done. I am officially marking this mystery solved. A piece of Whelen and warning equipment history that existed from 1987 until 1993-ish will not go lost/forgotten.