I am stumped. Whether or not Michael suspected Fredo, why trust him with all this sensitive information? If he was merely "weak and stupid," that should have been enough for Michael to know not to tell him...UNLESS he did suspect Fredo, and then it would be a matter of "keeping your enemies closer." If Michael telegraphed to Roth (through Fredo) that he knew about the assassination attempt, it could have thrown Roth off his plan to have Michael driven home. It was also a kind of declaration of war against Roth.