Looks like a pretty good list, although a little too restrictive for my taste. I personally have a more libertarian mindset, but I am not the kid's dad or guardian.
I grew up around guns and actually knew more about them when I was 12 than my parents did. I always knew where they were and how to load, unload, and fire them. I also knew where the ammo was kept. And guess what? I never abused my ability to access firearms and never misused them. Of course I also knew that I would be in HUUUUUGE trouble if I ever did anything stupid with guns, both with my parents and possibly with the law, so that was a sufficient deterrent in and of itself.
I now have kids of my own, and have observed in others (not myself, as I have very vivid memories going back to about 21 months of age) that the farther removed someone is from being a child, the more they tend to dismiss a child's ability to think, reason, understand, be aware, etc.
From personal experience, as others have said, having controlled access to weapons lowers the mystery and the desire to sneak around to obtain additional access.
It basically comes down to trust, your gut feeling, and personal maturity on an individual basis. At some point, people are either to be trusted to handle a gun or they are not. I think 14 is plenty old enough to have a single shot 12ga, especially if a responsible grandfather and BIL both agree. For some it might be 12 or 10 and for others it might be age 30, 80, or 2,000.
I would also add that if doing a written contract of sorts that I would write it in such a way as to make sure that it cannot be misconstrued as an illegal transfer, possession or storage of a firearm by an prohibitted person as defined by applicable federal, state, and/or local laws.