If Fredo didn't open the drapes, what exactly was his role in the plot? You say you don't believe that he would do that, but any other role you want to assign him, like letting the assassins onto the grounds, or promising to help them escape, gets him in even deeper. You have to believe that with hundreds of guests roaming the grounds that day, many of whom I would assume were relative strangers, the house would have been pretty well guarded, as was pointed out here in another thread by, I believe, Turnbull. I have a few problems with that scenario:

"They approach Fredo after the hit..." You're implying here that Fredo's men knew Fredo was involved. How would they know that?

"He orders his men to kill them without leaving a trace" So Fredo's men locate the assassins before any of the other dozens of men who are also looking, aren't seen locating the assassins by anyone else, kill the assassins without being seen by anyone else, and then dump them in a ditch without being seen by anyone else.

Not likely, IMO.

The complete assassination plot is never fully fleshed out in the film. Obviously, to kill two assassins would have required at least one (If Fredo helped) and possibly two other "plotters", who are never identified, nor is their role.

It's just a big black plot hole as far as I'm concerned.

You may also want to read THIS THREAD for a recent dicussions that adavnces the theory that Rocco participated.