Fredo had not been outed as the traitor in the family until the superman show

I believe, Fredo had plenty time and probably opportunities to tell Roth / Ola

• Michael had figured Roth was the one who engineered the Tahoe bedroom shooting, failing which Roth had now arranged for Michael to be killed in the Cuban military car taking Michael back to his hotel after the New Year party
• Michael had arranged for Roth to never see the New Year

It seems to me for whatever reason Fredo chose not to go running to Roth / Ola with this pivotal information “keeping the trust Michael had entrusted” when the brothers were having drinks

However when the senate hearing came around Fredo withheld all the information that could have helped Michael because the jealousy, resentment was back and the misguided allure of becoming Don Fredo - something in it for me - was too much pull to resist

Also if and once Michael got into that Cuban military car - Bussetta, Neri, Rocco – no one, nothing would have saved Michael!