SB, everything you said is true. It is hard to imagine Fredo consciously setting up his brother for a hit. But, as the Don said (in the novel): "Revenge is a dish best eaten cold." Few people hell-bent on revenge wait for the dish to cool off. Others hear what they want to hear, as long as they think they'll get their end. Roth and Ola may have "misled" Fredo in some way, but he was ready to be misled.
IMO, a key to Fredo's attitude comes in the boathouse scene. The anger, rage, frustration and resentment against Michael really boiled over in that scene. Viewing that scene again, it's not so easy to think of him as "sweet," "good-hearted," "harmless."