...Had Michael paid even the slightest attention to Fredo, Fredo might have been able to summon the loyalty to his family and report that he'd been approached. Armed with this information, Michael most likely would have foreseen Roth's moves and end game and avoid the fiasco in Cuba.
How do we know that Michael didn't pay 'the slightest attention' to Fredo? Just because there's no direct evidence of this, doesn't mean it didn't happen.
And again,
in what way could Michael have paid attention to Fredo that would have inspired Fredo, already resentful at having been stepped over...to shun Roth's offer of 'something in it for him' and report his having been approached.
If we are going to attribute Fredo's betrayal to a weakness of Michael's, then we should be able to come up with specifics instead of 'coulda/shoulda/woulda' generalities.
As mentioned earlier, hindsight is 20/20.
