Visible crew/equipment: When evil Debbie Jellinsky is electrocuted, two stunt wires/cables are visible on her shoulders in the first close-up shot.
Revealing mistake: When they first arrive at camp and Joel goes to use his inhalers, the mouthpiece is actually in the storage position rather than the actuation position. The puff sound must have been added in later because the inhaler cannot be activated when it is put together that way. (00:22:20)
Answer: They can be killed. In the first movie, Morticia talks about several relatives who were murdered, and the whole family seems pretty scared when they're strapped in the electric chairs. They're hard core and lucky, but not immortal.
Brian Katcher