I don't know about the rest of the questions, but I've stolen Dark Matter from Zeromus a few times even with Edge on level 60 (on SNES). Getting it seems to be quite random. Sometimes Edge steals it on his first try, sometimes he can't get it after 10-20+ attempts. I'm not really sure you need Dark Matter anyway. The times I managed to steal it, it didn't reduce the damage done by Big Bang at all, and I've heard stories how its effect is random: sometimes it reduces Big Bang's damage, sometimes it doesn't do anything, sometimes it even increases the damage.