You might test your theory this way: next time the light comes on, after you stop and turn off the car, move the seat yourself! If the light is off on your restart, you've got a real wiring issue somewhere. (Yes, you still have a sensor problem, anyway, according to the code.)
Did you have any work done on the seats? Are you the only owner? Have you noticed you need to replace certain fuses regularly, or you smell something *weird* in the car?
I'd say the on-again-off-again light may be due to a wiring issue; the fact that it is on at all (and your other problems) tells me you have a definite problem going on under the hood.
Once you track down a sensor and put it in, clear the code and run the car. If the light stays out but the overheating issue remains, I think you already know what we'll say. Have your compression checked at that point, if you aren't planning on doing it now to find out anyway.
Now that you know the codes can be read for free by any AutoZone or Schuck's in Washington (or any other location nationwide), if the light goes on do not wait
before having it read! Please. That light is there for a reason, and ignoring it can be costly. Ignoring one problem can turn into a cascade of others over time.
Another parts place to try: Aaron's Auto is a Subaru-only junkyard in Burien, not too far from Sea-Tac airport. Call 1-800-541-7271 M-F, 8:30 to 5:00 PST, if you need something you can't afford new, or can't find locally anywhere else. They have a website, aaronsauto.com, but calling is easier, faster, and more accurate. FYI: they don't take American Express.