First I would like to say this sounds like to say this sounds like a WONDERFUL idea. So...
1. Its worth a try. If it goes well maybe you can spend sometime prepping the boys for a more in depth campaign time permitting of course.
2. Thats a tough one. You don't want to have to exclude anyone because you have to many players. Try to incorporate as many as want to play, however if there get to be to many try a dry run with at most 6 players (preferably seasoned players mostly) and let everyone watch to get an idea of how it works. Obviously the short mine adventure would work great for this. Then you can see if one of the other boys wants to try their hand at DMing (yea I know this sounds like a bad idea, but the boys should be learning leadership skills and this is a great way to help build some of those skills). If you have to many who want to play also you could try having your slightly seasoned DM take a crack at running a side game to help incorporate everyone.
3. I would use the pregens, but perhaps use the ones in the box also as a template to create a few extras just in case you need more to save on time trying to create them at the campout.
4. Take the extras just in case. Be Prepared after all right?
5. Depends on how much interest you have for players vs. ho wmany are not sure and want to watch first.
6. Why not? If you do I would maybe try to get one of the boys who is interested in DMing a game to do it with your guidance so if your group does expand you can Be Prepared. Also would help if you have more boys than one group can handle so more can be involved if they want to.
Hope this helps. And again, FANTASTIC IDEA!!! If I ever get the chance to be a leader in one of our Boy Scout troops again I will have to try to remember this and maybe use it myself.