The stuff you're talking about in your spoiler, I'm completely on board with. It's all related to dramatic irony. Tess doesn't know Keith is a normal dude, she don't know AJ is a rapist, the woman in the past don't know the old dude is a creepy fecker... That's all ok. We all make bad judgement calls and ignore obvious red flags. I have no issue whatsoever with any of that.
My issues come with the moments of suspense (when we and the character are finding things out at the same time). It's these moments that the character decisions can not be moronic (especially if it's not a snap decision). The best horror experiences, especially in a grounded film is when we put ourselves in the characters shoes... You can't do that here.
Let's take the scene where Keith returns home as this is the moment where I, and many others I'm sure, check out of the film.
Tess is established as a smart and thoughtful person. You can see that the way she demands to see Keith's booking reference etc. And her conversation about the difference in his men and women have/are forced to behave in situations like the one her and Keith find themselves in...
So after getting locked in the basement and finding a dungeon where someone clearly was locked up in, and the fact that we the audience and her the protagonist don't fully trust Keith (by design)... He goes in there to check it out, she's waiting at the door. Ok fine. That's what we would do potentially.
Then she goes after him and nearly locks herself in. Like, after an ordeal like the one she just had, she would be more careful. But feck it, maybe she wasn't thinking straight.
Then she walks back into the dark area and Keith is nowhere to be seen. At this point, she don't know Keith, she doesn't trust him... Why the feck don't she leave? But ok, maybe she's worried about him... Sure.
Then ANOTHER door is discovered, with stairs leading down to darkness and this guy she and we don't trust is calling her to help him...
I'm sorry but even my 5 year old would know what to do in that scenario
There is absolutely no fecking way she, a smart woman who already has said that women can't be as brazen as men in these situations and was so hesitant to trust Keith at the start would walk down into that dungeon. No way at all. No one would do that. Ever.
She realistically had two choices. Either get out and drive away or get out and call someone. Maybe the cops, maybe a friend and tell them where she is etc.
NOBODY would walk down there! It's a decision that setup the rest of the film and a decision that is so idiotic that from this point, I just don't care what happens because I know there's no way 99.9% of the people on this planet would end up in this situation.
This is not a bad judgement call... It's a lazy way to get her down there and move the plot forward.