I'm sure they would. They do a dandy job preventing avalanches off a metal roof, and I suppose they could would work on a steeply pitched asphalt roof, too. One would have to make sure the snow guards are secure into plywood sheathing or rafters, and they are adequately caulked. I would only use them where there is a real danger of a lot of snow sliding off a roof all at once..
I have a metal roof on my garage and attached the snow guards only where they can be screwed into a rafter. Depending on the pitch of your roof, hundreds of pounds of snow could be pushing against the snow guards. Merely screwed into the metal would not be adequate support in my view and might even damage the metal if they would pull out. Perhaps a really good construction adhesive coated between the metal and gasket, and then between the gasket and the snow guard might work.
No they don't but you can order online and it doesn't take long to get here
Home depot has sub contractors that can install
Good question? I have not experienced anything like that. If you have debris flying around and hitting the snow guards you will most likely have a bigger problems than braking a snow guard.