A mess

And them.

3. I didn't like that part either, it was suggested that "every soul has two", or some bullshit like that. I figured that somehow everyone was born with an identical twin, one of which went to the underground.
5. The human chain was explained right at the start: the TV in 1986 was covering the "Hands Across America" movement in which thousands of Americans held hands from "sea to shining sea", and obviously, the Reds wanted to replicate it.

1. Probably repressed the memory of it. If you start living someone else's life long enough you probably could convince yourself you are them?

2. They were still tethered. "The soul remains one shared by two." The only reason the clone was able to switch and escape was because the original Adelaide went into the fun house. Some government scientist somebody carelessly left the door to the underground tunnel open one time? I'd say out of all of your points this one has the most potential to be a plot hole.

3. Red said "It was humans that built this place. They created the tethered so they could use them to control the ones above like puppets. But they failed and they abandoned the tethered. For generations the tethered continued without direction. They all went mad down here." So basically evil government mind control experiment with clones that were abandoned and the subjects all just went insane.

4. Red could understand English so maybe she overheard the government scientists talking? The other clones just made noises and weren't taught English.

5. Red says "I didn't just need to kill you. I needed to make a statement that the whole world would see. It's our time now. Our time up there." I think the idea was that once the clone had killed their human double that they then joined the chain. The dead guy being taken into the ambulance when the Wilsons first arrive at Santa Cruz is the homeless guy holding the Jeremiah 11:11 placard. Seems he was murdered by his doppelganger who then went to the beach to take his place in the chain (Jason sees him on the beach with blood dripping from his hands.)

The movie is very weird and ambiguous about a lot of things and leaves the viewer with a lot of questions but it did enough to satisfy rather than frustrate in my opinion. The mystery is always more interesting than the answers anyway.

My thoughts

1. “Adelaide” was treated for severe PTSD and her parents were doing anything they could to help her forget about that night. Remember, they think she just got lost in the mirror maze and freaked out. It showed them taking her to dance lessons like the therapist suggested.
I’m guessing they continued with the therapy on top of this and she just suppressed it. She was a little kid, desperately trying to become the real Adelaide so it’s not a stretch to believe she wouldn’t have forgotten.

2. She was locked up and scared. Because the tethered didn’t have souls, she was likely there for a while tied to the bed. She was again, a little kid and would have been terrified, plus she was nearly just strangled to death. We also don’t know that they aren’t being watched by whoever created them. They might have tightened security after they saw what happened. Clearly other tethered people weren’t leaving en masse otherwise they’d be public knowledge.

3. Some arm of the government. Like they’re cloning shit already. They cloned a sheep when I was a kid. It’s not that far fetched.
They kept appearing because Red organised them. She trained them and told them what to do. She let them know what they were missing and created a movement.

4. Adelaide remembered it. When they were on the way to the cottage she saw the now old man with the sign and it jogged her supressed memories. Then her son went missing and that jogged her memory. You see her become more aware of it as the holiday progresses. We also hear a little exposition with the news reporting on it etc.

5. Red I guess (real kid Adelaide) was seen watching TV at the start of the movie. There’s an advert for hands for humanity, a real thing in the 80’s. It was like a vigil I guess where people stood hand in hand.
She’s also wearing a t-shirt with it on before she gets the Thriller t-shirt. She was like 5 or 6 when she went down there. She obviously based her movement/escape on it.

They complain because a quality movie is a quality movies no matter the genre.

Really? You don’t get why people complain about things that don’t make sense?

The brother was still tethered to his clone, might have been something to do with them being the youngest. Maybe the older you get the weaker the tethering gets and it eventually breaks? Regardless it's not a plot hole.

They all had no chance of survival? Not sure what you mean by that as they all survived. Not sure how they made dumb decisions given the circumstances, the only dumb bit was after they all escape on the boat and they go to their friends house instead of a police station but they didn't know at that point that the doppelgangers were all over the country killing their opposite selves so I'll let that slide. The father was the real MVP, he killed his double and the double of his friend and he was a likeable guy, not sure why you were rooting for his death.

The acting, directing, musical score and the screenplay were all good in my opinion, we probably get horror/thriller movies this good about once a year if that.