I'm having a hard time following your massive paragraph.

Particularly I'm confused on which manifolds and heads won't work according to Nasioc, into which cars.
Also, what is broken in the wrx?
For the Legacy electronics to be tunable, you really need a $$$ standalone, and $1000 or more in tuning, to do what the modern WRX ecu is able to do (open source tuning with a laptop and cable). Swapping in OBD-I legacy wiring is certainly possible as a wiring god, but you lose all the advantages of easy and cheap modern tuning.
My advice, is to pick a car you would rather work on, set specific goals and save about $3-5 grand for, then do the swap or repair.
Your WRX is running a WRX ECU, not STI as those aren't compatible with your WRX heads.