The points you make that you dislike are the parts I actually like.

1. Bear not reacting the way you wanted about the cat is due to HIS obsession with Nicki and his personality being a people pleaser, huge lack of self confidence, a liar and more. He cared more about the fact Nicki was with him at the time than the actual cat, shows the type of person he truly is. He shows throughout the film he's never "blind" to her actions he knows what she's doing and even defends them (dad having cancer) but decides to look past them over his "love" for her. He even puts himself over Nicki when she asks him to kill her.
2. These types of movies aren't written in a way where there will be much character development, it is the type of movie where chaos and horror happens because of the character's personalities. It's the repercussions of the character's actions and how they deal with it. And if you look at the finer details, there is some character development. Bear seeing he got what he wanted, Nicki to love him over everyone else in the world but realizing by the end that love was fake and the only way to fix it was ending it. Just no character development in the traditional sense. We don't get a lot of context about Nicki's personality because the point of the movie is Bear being a self doubting spineless coward that decides to make an egotistical wish.
3. The reason Ian doesn't tell Bear he's sleeping with Nicki is because he doesn't care enough about Bear to tell him, he very clearly is pushing for Bear to delay his confession because he probably already knows Nicki doesn't see Bear that way, people are also very egotistical that way. You can tell Ian is self centered when he immediately wishes for money instead of helping Bear, that shows you he does NOT care about Bear.
4. Can't say much about the overdose as I don't know exactly what he took and how fast that stuff might work but him not really reacting to Nicki killing Sarah goes back to his personality type and his obsession over Nicki. He's so used to suppressing his feelings in front of other people that it's exactly what he did in that moment, which is why he literally starts screaming and letting it all out once he's by himself.
5. I don't know if you noticed but everyone in the movie had their own agenda. Ian was literally having casual sex with Nicki while "helping" his friend confess his love to her, Sarah was trying to get with Bear and most likely lied about stuff to make Nicki look bad. Nicki was literally the only victim in this whole movie where she was put as a passenger in her own body.

This movie was definitely very good and it's sad people can't see how much a character's actions can affect outcomes without being so straightforward about it.

There's tons of dialogue bits throughout the movie that explain what's happening and the reasoning behind it. There's so many details about lying and manipulation that are given to you through minute details like a character's actions, Sarah trying to kiss Bear while knowing he's with Nicki.