My two cents worth! All sentiments aside...

No alarm bells for me either regarding "Havana's great, it's my kinda town" and later knowing the local drinks, local entertainment does not necessarily mean to me that Fredo had been to Cuba before

How else is he going to show people a good time without local knowledge?
Besides one call to the hotel concierge and he would have had all these information

Vito, Michael, Tom, everyone even well-versed Godfather scholars of this board! underestimated Fredo that he was so dangerous and detrimental because he was stepped over, for his kid brother

Nobody not Michael, not even Vito had any inclination how detrimental Fredo would be, arguably even setting Michael up to be murdered, for the Donship
Nobody detected in Fredo the potential for betrayal but in all fairness how and why anyone would ever even suspect blood family ever

Turnbull put it beautifully!
Originally Posted by Turnbull
He was such an ineffectual dunce in GF that his betrayal of Michael in II had strong impact--we probably never would have guessed he had it in him...and what else he might have tried if Michael had given him a pass. Guilt and gratitude are the most fleeting of emotions

It could be that Fredo had already been sent to Vegas before the old days Ola mentioned and "We never met." at the hotel or anywhere
He said “Johnny Ola bumped into me in Beverly Hills”

Fredo was constantly sidelined by Vito and Sonny, long before Michael took over but of course he would have never done what he did to his kid brother, to his father or older brother

Vito also figured that Fredo was too weak and stupid to run the family but also didn't figure being stepped over, for his kid brother would cause enough resentment to turn him into a traitor

I reckon Vito is the one who was most careless, destructive in not picking up on disastrous stuff out of the three even slipping regards Sollozzo who was not family unlike blood Fredo and Carlo who was also Sonny's friend prior to marrying Connie

Roth and Moe Greene connection had only just surfaced and I reckon Michael seemed pensive perhaps even suspicious when Fredo / Ola introduction was made, may have started to put the treacherous puzzle together but still in denial?

Brothers! Fredo arguably setting Michael up to be murdered, for the Donship Michael the Don actually murders Fredo