I think the problem here is that we interpret the story from the end. We know already who the traitor is. So every hint to him appears large. All the hints to others appear small.
(For example, if FFC had given the story in GF2 another turn, say Fredo dies in Cuba, I bet that every-one here would say, that Michael could never ever kill his own brother.)

We never get to know why Tessio is supposed to be the smarter one. Obviously, he wasn't smart enough to stay alive. I doubt that Clemenza really trusted Vito's judgment at the point in the fish tank scene. Clemenza is convinced: "Mikey, you're wrong!"

Why couldn't touch Barzini Rocco, when it was easy to get in touch with Tessio?