>>1819318It's an aluminum frame so it's fine. Just put a protective sticker on your frame in that spot.
>>1819714Oatmeal cooked in milk with peanut butter, sliced bananas, chopped dates, and molasses.
>>1819775Apply penetrating oil, clean as much debris out of the extractor threads as you can, thread the extractor into the remaining threads as far as it will go - then get a big adjustable wrench and make sure it's in as far as possible. Then cross your fingers and try to remove the crank. If you fail then you'll want to find a gear puller (pic related).
>>1819846Some light oil on the pivots wouldn't hurt, but I'd guess your hanger is bent.
>>1819906Yes, some early MTBs used the cruiser/Schwinn 21.15mm quill size, narrower than the more common 22.2
https://www.sheldonbrown.com/harris/opc.html#stem 