1. Dunno about specific mentions of +P ammo in the manual, but I assure you, it is ok to shoot.
2. AFAIK the chamber is fully supported. I've noted no bulges in the cases like I have with my Glock 20.
3. CCWing with a fullsize would be harder than a compact, but it's not bad. The only dimensions that change between M&P Fullsize and M&P Compact are the barrel length, the grip length, and the beavertail (eliminated on the compact). I'm reasonably slender, so the fullsize gun does not quite disappear under an untucked shirt, but it is not hard at all to conceal if you wear an unbuttoned shirt over a T, or a jacket.
Mike