While dealing with the crazy injector issue I've currently got going on, I've came across another perplexing issue...finding my ECU! I have both a Haynes and a Chilton's manual and both insist that my ECU is under the carpet on my front passenger-side footwell, but I'm not seeing anything. Ideas?

Yup, haynes & chiltons are wrong for the first gen legacy. The ECU is located up under the kick panel in the driver's footwell to the left of the steering column. Here's a pic

 

http://main.experiencetherave.com/subaru/images/ecuhide.jpg

I thought that was what those might be. Any other oversights worth mentioning in those two manuals?

try investigating the injector resistor, the ceramic piece encased in aluminum that bolts to the passenger side strut tower.