If the charge is already in contact with the base of the bullet, it might not take much setback to compress the charge ... if in any doubt at all, measure OAL of your carry rounds ... compare against a few fresh out of the box. It can't take more than a few minutes.
Maybe. But less so if it's not the top round in the magazine having been covered by another as magazines are switched and chambered rounds replaced in the magazine in the gun belt pouch rather than always removing the mag from the gun and topping it back up.
I've done it myself, with Browning GPs and M1911s, and others.
May not take much change in seating. +P[+] ammo (presumably used for serious Los Alamos type security) is already stuffed to the gills, so slightly deeper seating may have a significant effect.