TB, I doubt that Fredo or anyone gunning down the gunmen amidst the cacophany of machine gun fire doesn't work. The gunmen were found a distance away from Mike's house in what appears to be a drainage ditch. The geometry doesn't work.
Also, Fredo in a crisis? Let alone with the job to murder two people?
According to Deana, the gunmen were killed somewhere in the proximity of she and Fredo's house. "Right out my window! I want to get out of here! They're lying there dead!" Fredo is rather frantically dragging her away, assumingly trying to keep her quiet(?)
And I don't think it's a stretch to think that Fredo had an assistant. I think the two of them could have overcome the seemingly non-linear geometry of the situation, dragging the bodies from outside his (Fredo's) window to the nearby drainage.
