two questions (ceiling fixture and clamps)

I've been rewiring my house (getting rid of K&T, and running extra circuits [one 20 amp circuit on 14 gauge K&T, powering up exterior lights, living room, dining room, hallway, 3 bedrooms, etc. Not good]). I've came up with a couple questions during the process:

1) When running wires into the circuit panel, I haven't been using any clamps (never thought of them). I see other clamps that was installed before, and they appeart to just be a small opening with screws around to hold the wire tight, but they require inserting the wire through them first. Are there any clamps that are designed to be put onto the wire, after the wire has already been ran? It's not a huge deal if I need to detach the wiring first, but I do like convenience.

2) The ceiling on the 2nd floor is 1/2" drywall, backed by 3/4" pine plank (old house). You can nail a fat woman to any spot you choose, and that ceiling doesn't move. I was wondering if fixtures exist, that allow mounting a ceiling box directly to the ceiling. If not, then I'd have to try to saw a hole out of the plank to access the ceiling joists.

Thanks for the help!