Most of your questions have been discussed here previously. We can only speculate what Fredo might have done, because the movie choose not to explain. He may have been the one to open the drapes in Michael's bedroom, or he unknowingly gave access to Michael's house to the person who did open the drapes. We know Fredo was dissatisfied with his place in the family. In the deleted scene were he and that trampy wife arrive at Anthony's party, trampy wife says "why are you afraid of your own kid brother"? The look on Fredo's face when he replies "because he's the head of the family" says a lot about his anger about his place in the family. He lied about knowing Johnny Ola. We don't know who killed the hitmen. I doubt if it was Fredo and so on and so on and so on.

Do a search and you will find many topics on this subject.

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Yeah, I left it noisy, scares away any pain-in the-ass innocent bystanders.