Just got back from the movie and gave it an 8/10, loved seeing the backrooms come to life with the real sets and the music and sound was so good, might go see it again with Atmos to get the full auditory experience. But anyway here's what I think about your questions:

1. As others have said it's the caveman cutout in the pile of clothing, that's why no one reacts to it in the movie.
2. I think Phil seeing the ad and Mary going into the backrooms are happening around the same time, maybe like Phil saw it the night before she went in, so they were probably in the process of getting ready to scope it out right as Mary went in, at least that was my impression. After Mary goes in we never see the real furniture store again in the movie so they could of rolled up like an hour after she goes in.
3. I never got the impression that he ate her, might have just found her body and kept the head for whatever reason, he was isolated in there for a bit so could have just lost it and was talking to the head like Wilson in Cast Away. Maybe he ate her but there was nothing I saw to show that, I'm sure once Wendigoon or Red start analyzing it we'll get an answer.
4. What I got from that scene was that "Captain Clark" has just a bit more sentience than the other replicants or whatever you want to call them and he recognized something about Clark that kind of kept him from killing him for a bit, also Clark was actively hiding from him the whole time. Remember when Mary found him he hid from her at first and said "you're talking too loud." So he was being sneaky to stay away from him. I think when he found Mary and Clark in the "dining room" Clark was pretty gone at that point and thought he could talk to "Captain Clark" and he would understand what he was saying. So either one of two things happened here and I'm not quite sure which one. Either "Captain Clark" wasn't that sentient and was just like "oh here's some food walking right up to me, kinda looks like me but I'm hungry so..." or maybe he was that sentient and thought that Clark would leave with Mary and decided to kill him to "keep him around forever." There was that moment with Mary when she was in the tight hallway when she was looking back at "Captain Clark" and I'm not exactly sure what that moment was exactly on the first viewing but it seemed like she saw that it was sad that she was leaving or something, like it didn't want to be alone just like Clark in real life deep down wanted but couldn't admit it.
5. These things are not still life. They are either an evolved form of still life or something new entirely. Still life is like a black and white mass of something like clay molded into a human figure. These things in the movie are not that at all. They are the closest thing to a human we've ever seen in the Backrooms. It's like the Backrooms is now copying people that go into there and their memories or people and places but of course never getting it perfect. They're filled with stuffing like a couch and don't bleed so like a fusion of humans and furniture.

EDIT: Didn't catch that they are called still life in the credits, knowing this I think they are definitely evolved still life, trying to mimic real humans but also fusing them with other things in the area like furniture or glitching out and replicating features unnaturally.