1. That'll happen. Hard plastic+temperature change=cracks. That's my only real complaint about my car... They do make plastic/vinyl repair kits, I've been meaning to get one for my car because the crack on mine reflects in the windshield and bugs me when the sun is just right.
2. Pull the switch. I've seen some power window switches that partially melt, that could potentially cause both your problems. Otherwise, sounds like somebody F'd with the wiring.
3. You don't need the expensive wiring harness unless you want to keep the warnings for headlights, keys in ignition, etc. You can just get a regular adapter pigtail and wire that the the pigtail that comes with the stereo. If you do it yourself,
read this. It will save you a trip back here later.
4. Stain remover?
What do you mean suspicious?
Some people will argue about the answer to your first unnumbered question. I'm an automotive technician and I drive a Cavalier, and I have worked on almost every car sold in the U.S and know lots about them all. Not saying the Cavalier's the number 1 car in existence, but for MPGs... That's all I'm going to say about that. If you take care of it, easily 200k miles. Flush the transmission and P/S every 30-50k depending on driving, I personally recommend and use BG's products for all flushes. Flush the coolant every 2 years, and put a can of "44K" in the gas tank every 44,000 miles. The only real problem is the wheel bearings (hub assembly) that tend to go bad. Yours were probably replaced around 60-80k, so you'reprobably good there until 150k if quality ones were used when they were replaced. When they do go out, replace them with Timken, they'll usually go 100k. I've seen some of the cheap replacements go out in a matter of months, and the dollar difference is usually less than $20 between Timken and the cheapest.