Did she cause the accident?

No, definitely not. Putting aside the clearly shown fact that he was driving an unsuitable car in a blizzard on a winding, narrow mountain road and couldn't see where he was going, why would she have risked killing him at that stage? Going off a cliff in a blizzard, you can't predict what kind of injuries will come out of that. He could so easily have been dead before the car even came to a stop. She's had medical training, so she knows this well. And she had no reason to hurt him at that stage. But she was definitely stalking him, and she took the opportunity to trap him in her house and make him hers. In fact it's possible that she didn't even have that idea at first but genuinely just wanted to help him, and then once she realized he was in her power she decided she would make him belong to her from that point on.

Had that blizzard not happened and Paul not gone off that cliff, it's very possible that once the book came out Annie would have deliberately kidnapped him the next time he came to Colorado (as he inevitably would have). But at the time of his accident, she had no reason to do such a thing.

I assumed she did

No one knows. But when they do the remake, it needs to be very clear that a white man causes the crash of a black woman who is the author of Misery and holds her against her will

No. She admits she was watching the hotel he was staying at. She just happened to be there when he left. As far as the accident. Well, weather for one. Sheldon admits he had no clue of the storm. He had just had some champagne. And he's a NY guy, not used to that kind of driving. NO, it was an accident. Annie was watching him and close by.