just change the spark plugs yourself, its a no brainer. if you're not confident with that kind of work, just get a random friend to help you. its very very easy on your car.

the thermostat, while a bit more complex, is far from rocket science so you shouldnt be stressed out there either. holy cow i just came up with something...

okay here you go:

because your thermostat is stuck open, your engine has been running tooo cold, or been exposed to non-operating temps (cold) for prolonged periods of time. many consequences can occur because of running too cold. i'm rusty on this stuff so if anyone can chime in please do.

first, your piston rings wont seal as good as they should because your pistons/rings/rotating assembly were designed to expand/contract and optimally operate within certain temperatures. this will cause some blowby...(let me know if u need to know what that is)

second, your ecu will think your car needs to be "warmed" up, and will run rich for "coldstart" conditions. while not exactly harmful in the short term, excessively rich conditions will eventually foul out spark plugs, oxygen sensors, and catalytic converters (possibly why your HC's were so high, even though your engine may have been warm).

good luck sir.