I agree with much of your assessment. I think where it's unclear; reading much of the responses is the drapes and their implication to the guilt of Fredo. I do believe that there is a connection between the drapes and the murder. I don't think however that Fredo would consciously have participated in a plot to kill his brother. He could have far more easily been duped under the perfectly common notion that a long time friend of the family and his father's wanted access to the compound to talk to Michael for the benefit of negotiations. A conscious betrayal contradicts what we know about Fredo from the first movie and what’s established in the second movie and the book. Even after in the movie Fredo is still good hearted with Anthony, going fishing. Still the same old Fredo. Part of the emotional climax of the movie and the bitter taste afterward is the absolute horror we feel for Michael having murdered his own brother. I think this is complicated by the fact that we feel for much of Fredo and sympathize because of his nature and weakness, to us Fredo is more like a child than a man, he has a sorta innocence about him.

Where I am arguing however is the drapes issue. Because if the drapes were left open by Fredo it would imply some level of malicious intent or conscious knowledge that something was up. Meaning he would have indeed betrayed his brother. Having watched the movie and acknowledging the grey spots in the plot (which I think are great due to the fact that they allow us to know what Michael knows, maybe a bit more) I think it was FFC's contention to have a movie where a ridiculous scenario played itself out, one where Mike's brother apparently betrayed him but due to Mike's tyranicism and cold nature he would believe it was a plot and have him killed. I acknowledge Fredo fucked up, but Fredo having allowed the blinds to be open would be a different story. I am not discounting that possibility, I guess to a certain degree I don’t want to believe Fredo was capable of opening the blinds (LOL, it’s so simple but so significant). But looking at how vague things are left, I am also acknowledging there could have been another possibility.