I was kicking myself in the ass for messing with the FPCM -> FP wiring in the first place after I started having all the problems, thinking it was something I had screwed up. Even if you look at the Impreza plug wiring, it's still quite a bit smaller than the overkill 10awg wire I'm using. It's probably closer to 14-16. If you can snag that plug out of a junkyard, it would probably be even cheaper than ordering an entire harness from Subaru - but it makes the wiring literally plug and play if you get some step-down butt splices.
It's just my luck that I've had this much trouble with the car - it's been a nightmare since day 1. If this had been my first experience with a Subaru, it definitely would have soured me on the brand.