See page 61 of the rulebook for armor penetration. It says if you get a 5, 6, or more the vehicle is destroyed, which means you don't have to destroy every side.
And yeah, you strike in initiative order even when you charge, unlike fantasy. Also unlike fantasy, you get +1 attack for charging.
I do not understand your third question. Units fight units in assault, it's not a model vs model thing like lord of the rings. All 5 orks attack, but they can't chose which space marine they are attacking. All 3 space marines attack as well.