Regarding Fredo and the drapes.....

I think we have to give more weight to the evidence of what we are actually shown in the film, rather than what we can speculate about.

We know Fredo was involved in the plot

We know someone opened the drapes.

Someone = Fredo

Fredo is the only plotter identified, and the only action he can be identified with is the drapes-opening. I think we can all agree that you can't identify Fredo with the murder of the hitmen.

Any other elements in the assassination plot (how the hitmen got on the grounds, who killed them, how the killer got away) are pure speculation, and to associate Fredo with any of them is equally speculative.