I wonder if Fredo really had men of his own or if Fredo's men were really just Corleone men who would do reasonable Fredo biddings- and for anything other than reasonable biddings they'd consult Michael, Neri or Rocco. I'm not sure I could see Fredo's men being comfortable in assisting in the assassination of the don- on Fredo's order.
Difficult. Not impossible...
It's the only explanation in case Fredo would have been up his neck in the conspiracy. I won't deny that it seems a little flawed. But I can't imagine that FFC or Puzo wouldn't have noticed these supposed plot holes themselves. Maybe they didn't care, which I doubt, or maybe they deliberately left it open to the viewer's own imagination. You can compare this with Once Upon A Time in America, which ending also leaves the viewer with a lot of questions. Leone, the director, apparantly did this on purpose so the viewer could make his own interpretation. The same applies to 2001: A Space Odyssey.
