Logically:
--If Roth wanted to have Michael killed in his bedroom, and...
--the drapes had to be opened so the gunmen could have a clear shot at Michael, and...
--a security guard was bribed or planted to open the drapes, then...
What would they need Fredo for?
I think that it's a very reasonable inference that Fredo opened the drapes.
When that morphs into Fredo must have opened the drapes, and from there Fredo must have known it was going to be a hit, and then Fredo must have been promised to be boss, then I think the inference breaks down. It ignores that Fredo says he didn't know, that Michael says he knows Fredo didn't know, that a generation later Kay doesn't seem to have resentment that Fredo tried to kill her. It also undermines the real tragedy of the film, which is that Mike's own rigidity is leading him to murder his own brother - over a screwup.
