Somehow I missed that your positive retention and shear pin questions were for the same place! I'd put a hole completely through the body tube and nose cone shoulder. Interference fit in the body tube (and I'd try to thread it); slight clearance through the nose cone shoulder. Install before flight, push the stubs through the shoulder after flight.
Take a look at the force required to shear the pins, and the pressure you need to get that force, to decide the size and number you need.
I don't like ejecting the body tube with the parachute, though I've seen it done successfully. It relies on inertia to eventually pull the chute out of the tube -- I like a more positive ejection, which is what you get when you eject the nose cone.