I also think the final decision was made in the boathouse. When you listen to Michael talk to Fredo and watch his body language it's apparent that he's through with Fredo. It's really good acting. Michael is polite, calm and cold as ice. He's already steeled himself for what he will do.

I really don't think that even then Fredo realized the extreme danger he was in but even if he had his resentment at being "passed over" wouldn't allow him to shut up.

It's unclear as to what exact actions Fredo took ( I can't see him creeping into his sister-in-law's bedroom to open drapes) but he's angry at Johnny Ola when he calls ("You guys lied to me!") so for what it's worth it's probably true that he didn't know about the murder plans.

I think Michael's Havana reaction to the discovery of Fredo's treachery was very emotional and very human. I don't think he had decided at that point to kill Fredo although he was obviously angry/hurt enough to consider it.