So how did the killer stab Esther?

How did Faith stab Esther when the lights went out? We know Faith caused the lights to go out and she's the one who wore the mask at the top of the stairwell (Gary says so when they find it in the cave and she also admits to knowing about it before killing herself). But how could she have stabbed Esther as from that moment after the masked figure appears the stairwell is occupied by Gary and older man who has a heart attack so she couldn't get back down? My guess is she did it via secret passage because there are multiple entranceways to the underground cave, though I don't think it's ever explained.
And when did she have time to kill Jenny's maid? The maid goes upstairs while Gary's interviewing the two young lovers in another room right before the lights go out and from there on Faith is "locked" in the room with Esther. She might've had time to kill the maid and move her body to the underground cave (not sure how much time passes from lights going out and the clock ringing it's 5 o'clock) but not without someone seeing unless there was a passageway that lead directly into the room the maid was in. But I don't think that's ever explained either and the fact that she never reappears after the lights go out and a shot's fired seems to indicate she was already killed. But there was no interval for the murder to take place.
Thanks for anybody with answers or theories. I really enjoyed this movie but after going over the key scenes several times I can't find the answers to my questions.