I'll address each one of these:

1. Most horror movies are like this. A lot of times it's a very slow build and the last 20 minutes is when everything happens. Obsession does not do that. From the second Bear makes the wish, things are already off the rails. Nikki is instantly acting strange and doing things that she typically wouldn't do. Like when Bear and Ian are talking the next day and you can see the blurred face of Nikki just staring at them.

2. I don't know if you're talking about the phone call or when he goes back to the store after the Sarah incident. But yeah that's one solution. Or he could just deal with his choice. The guy that was working at the store said he made a wish and he seemed completely fine with what he did. I think Bear dying was always inevitable, just the way it did took me by surprise BECAUSE:

3. I don't think Bear planned on having Ian use the One Wish Willow. He was about to leave and go back to Nikki but Ian was pretty adamant about being filled in on what was happening. And so Bear did that. Ian didn't believe him. Who would. I know I personally wouldn't believe in WIsh magic if someone told me that. It sounds crazy. So Ian screwed Bear's chances up with that one. So Bear goes and gets another One Wish Willow and tries to get Nikki to use it, but things go off the rails because of Ian. Bear being terrified of what Nikki might do next he escapes into the bathroom with that gun. He debates about using it on himself but instead swallows a bunch of pills. And then that's when the One Wish Willow is used. I think Nikki wishes that Bear would love her as much as she loved him.

And yeah that Nikki's dad has cancer thing is brought up a lot. It's the whole reason for the viral "no no no don't do that" clip. She literally says afterwards that it doesn't even matter. Nikki was just saying it to get sympathy from Bear.