fuel filter won't hurt anyway, but any I've had issues would start bucking rather than just slowing. But the filter would be worth replacing. Sitting that long, the gas could be bad too. Run it down if you can then top off with some good gas after you replace the filter.
Assuming it's an automatic transmission, did it downshift when going up the hill?
on the heater cable, assuming it's like my base model . .empty the glove box and pull the soft plastic in enough that the stop will clear the top of the dash so you can pull it all the way out towards you. Then you can see the heater box, may have to hold up some insulation. On the top you should find the other end of the cable hooking to a rod. Try and move it by hand, hit it with some WD-40 or similar and try to shoot some up the sleeve on the cable. May just need freed up a bit that way (mine was crudded badly when I bought it). Getting to the other end (behind the controls) can be a hassle, not real hard though. Try the easy end first.
the horns are mounted between the grill and radiator. You can disconnect them at the horn for now, but tape the wire ends so they don't short out on something. On a factory steering wheel there's two small hex head screws on the back to remove the center portion. Pull it and see if something doesn't look right (been a long time, can't describe much inside the cover).