why reach to open the other window ???

Agreed. I guess he was trying to see if she could have jumped from the bathroom window to the next window and might be lurking in the main room again, but after he determined the window was unlocked, that's all he needed. Why try to raise it up quite a ways? Definitely a weak moment in the movie devised to keep the plot moving.

This was a "decent" effort by the Coen brothers. Not a bad movie for being one of their first, but this is definitely overrated. I don't mind a slow burn, but this movie was an extreme slow burn. I'd give it a solid 6/10

To get his hand on the knife and free himself, which he did, unless I'm misunderstanding the question.