That's a good theory, but what are the chances that if you built up all this debt, that you're not going to continuously accrue new debt while you're paying off the old ones?

I've seen a lot of these disintegrate and separate, sometimes leaving parts inside the combustion chamber.  I'd stick with something more reliable...

Clean your MAF...probably won't help much but likely won't hurt.

O2 sensors usually get lazy...they don't throw a MIL but they just don't respond like they should.  I'd be willing to bet that's a big part of your mileage loss.  As for checking them, I don't know...I just always considered them maintenance items.

Have you considered seafoam?

They moved the light to that position in 2004.  The 2003's have it somewhere else (on the instrument panel I think?)