**Happier update! **
Well, it looks like the FrankenSubaru is alive. These are the latest updates :
-I got the new ECU a remanufactured unit by Standard of Canada.
-Following advise, I changed the oil and also flushed the cooling system and replaced it with ready mix Coolant.
-I completed a brand new wiring installation for the radiator fans. This included fixing the auxiliary fan (for the AC circuit) and the wiring for a main fan with a separate relay, yet both share the same signal from the thermo switch, making them start simultaneously but fed from different sources. They work spot on, by the way, the thermo switch is for a Hyundai that opens at lower temperature than the standard Subaru switch boiling hot head gasket killers!
-I found injector #4 leaking again. It was working but there was a leak around its base. FGS! So I got another injector from the junkyard. This one worked fine.
-I found green corrosion on the connector of the throttle position switch. I cleaned it both connector and leads. And did the same for the MAF's connector and leads.
With these fixes done the car started right away and to a somewhat smooth operation. So I worked on:
-Idle adjustments, this was really a PITA, that included swapping a MAF, throttle cable adjustment, idle screw adjustment, ignition timing, new rotor and distributor cap.
-Next is a second "field test", as I went for a spin around the block and noticed that dreaded hesitation and sputtering when going uphill. At this point it SEEMS better and more stable at idle, and when ramming the gas.
Early conclusions:
I'm yet to really know what was the real problem in the first place. I went through so many systems and so many fixes, that I just wonder if this car is so sensitive, complicated and badly aged, that it developed all these problems.
After all tests I'm overdue to take it for the mandatory yearly inspection so it's street legal. That'll be next week most likely.
Early thanks to all the great USMB members that helped me out! Special thanks to @idosubaru @DaveT @naru2 @john in KY
I'll post more updates soon