Ok,
so i guess it's probably miles away... or not. mmm, all beltpack/base station etc etc, were connected together via 3 pin XLRs. They called that Party-Line.

Try to draw a schematic of your existing tester board. It may reveal some interesting stuff. Same for the transistor board on the headset.

Sure it's doable... If it worked.. it can work again.

Don't give up!