Re: Point #3 in the initial post. The warmers work incredibly well when used this way. Some people swear by sleeping with a bottle of hot water between their legs, but the chemical warmers are better, in my opinion. They pretty much stay in place. I used one a few nights ago. It was a 'Stay Warm' brand, 4x5 inches, advertised as good for 24 hours. I was shivering, standing in the snow on top of a mountain, watching the moon rise and then the eclipse. Within minutes I felt warmed from inside out. Hard to explain, but the effect was dramatic.
I used it for about 15 hours (overnight in the tent) and left it on the counter when I got home, still very warm to the touch.
I've heard of re-wrapping these things (taking the oxygen away) and re-using them if they're still warm, but I wouldn't want to depend on a used one.