This is the correct MAF-T: https://www.extremepsi.com/store/MAF-Translator-Mitsubishi-Eclipse-90-94-BLACK-FRIDAY-SALE-FTS-050DSMTRANS1G.html
I wouldn't bother with it, though. Megasquirt is a way better long term solution.
I think the 88+ diff has a different axle spline count; 28 vs. 25 for 87 and older, if I remember correctly. I think I remember hearing that the axle parts can be mixed and matched (i.e. use parts from both 87 and 88 axles to use an 88 diff with 87 hubs or vice versa) but I haven't tried that myself and wouldn't want to bet on it working, sorry.
I have one out of an 88-89. Diff, torque tube, and axles. Unfortunately, I don't know much about it since I didn't pull it from the car; I got it with a load of other parts. I'm up in Rochester, MN and would be willing to meet you halfway to eliminate shipping. I can send you pictures/videos if you're interested.
There was a guy in one of the Facebook groups named James Kane who modified a Montero sender to work. Of course, because this is the way Facebook works, all the info appears to have been lost, so I can't tell you how to do it, but it is possible.
The C-clips hold the studs in the hub too! For some reason known only to Mitsubishi, the studs aren't press fit into the hubs like most cars, and if the clips aren't installed, the studs can slide out and spin, which sucks. I used Loctite 680 on mine to hold them in so that I could eliminate the clips and run aftermarket wheels.
Yes, the clips do go between the rotor and the wheel. The stock wheels have counterbores at the lug holes to clear the clips.
Sure, RockAuto has them: https://www.rockauto.com/en/catalog/chrysler,1988,conquest,2.6l+156cid+l4+turbocharged,1062470,steering,power+steering+pump+seal+kit,7407
You can get enough current to weld out of a 12V lead acid battery, which is more dangerous than a current-limited power supply if there might be a short in the wiring you're testing.
That being said...I still just use a battery. Good luck.
Turbo Cary's advice above is great. Also, given all the problems you mentioned, I can't help but wonder if you have a problem with your ETACS module. I'd start troubleshooting there.