Yea, in NO way is a "slit in the neck" visible on either of the assassin's. It's simply too dark to tell, and I doubt FFC would even put that in that scene as it makes no sense. Both assassin's were found in the water drainage tunnel and had to be fished out, we weren't shown any wet soldiers in any of the scenes or anything, so the whole garroting thing wouldn't make sense nor fit in that scene.

Fredo WAS the inside man, and I think Fredo WAS capable of killing. In the novel, Fredo was once the toughest of the three brothers and was looked at as a capable brute, that was until Vito was shot and he was so shocked from it all, that he had to be drugged up and was alone in his room for days. That whole situation was said to change him, in the novel. However we see in II and even GFI to an extent that Fredo can still occasionally revert to some traitorous, typical mafiosi, all about self, tendencies. I think him killing the assassins to keep his role in the scheme secret, is perfectly plausible. Not saying thats really what happened, but I think it's more plausible than the Rocco theory.