Wrt the mirror, it's not a perfect system, but I was thinking more in terms of "give a split second to where they see their necks sort of align with the jumpsuit" and then they jump out of the mirror and scare them as just a jump suit with a black hood for a head, instead of trying to find a way to make it the doppelganger while the actor is in motion. If the actor is holding still, people might think they're a mannequin and actually try to align their necks with the hole themselves, before they jump out at them. Again, as long as it's too fast to scrutinize and the actor is average height, people won't sweat it too much and they should be able to get a good scare in.