Of all of Michaels' men, I feel that neither Clemenza or Neri would have betrayed him. Neri was gotten out of jail for a murder charge when he was a cop. Clemenza, you you are how,he felt about Mike when he was showing him the restaurant gun in his basement.
As I have said before, Neri didn't want to kill Fredo, but had no choice to Do so on Mike's orders.
The theory of shooting him on the lake with the shot echoing all over the place - bad decision. How do you explain a gunshot to the back of someone's head "that drowned".
Neri could have whacked him over the head with an oar, " He stood up in they very front of the boat to pull in a big fish,lost his balance, fell and hit his head on the side of the boat and fell in
I got up to get him, but he already went under". Reach your arms into the water almost up to your shoulders and that's it. If the body is retreived, there's a cut on the head, case closed. Forensics in the late 50's when ever this happened was not like it is today.
In the scene where Nero pulls the gun out, there's hills around the lake, too many possible witnesses to at least hear that shot, let alone heard it then be looking around and see 1 boat on the water and someone dumping a body overboard.