yeah, that arm thing was ridiculous. I think it was only put there so that it would be plausible for Piper to fight off like 7 demons all at once. As for who the ghost voice is, it's not a single person.
Recall that when Leo asked, "what do you want", the voice replyed "what do we want? We want you." So whoever they are, they must be a group. I don't think it's Wyatt or Chris.
WHoever this group is, they are very powerful, to be able to shield a place from the elders and really just jerk everybody around. It almost makes me wonder if they are that group of people who asked cole to join them? Remember them? They were supposedly above good and evil.
Xin Li