Well put TB.

I would add to this the fact that Michael would have known that if Roth was using Fredo, it was not on an operational basis because Fredo was too incompetent. Further, after the Tahoe shooting when Ola made his famous "wrong number" call to Fredo in the middle of the night Fredo made it clear that he didnt want anything more to do with them, and they had got him in deep enough already. Fredo being Fredo probably saw the wagons circling around him. He could no longer help Roth, but he had to be afraid Michael would figure out that he had been involved with them. AGian, I think Michael suspected Fredo all along, but for many reasons he just did not want to believe it was Fredo. Its interesting that Michael knew Pentangeli was not involved and used Pentangeli as sort of a "straw man" when dealing with Roth. Clearly he wanted to make Roth feel comfortable enough that Michael was barking up the wrong tree on who ordered the hit.

I think Michael set Fredo up with the "did you ever meet Ola?" question, and when it became clear Fredo was lying we see the emotional reaction during the Superman show. Michael now has proof and knows he will have to kill his own brother.

I still think the disclosure about Roth wanting to assassinate Michael was done to draw Fredo into his confidence. On the chance that Fredo would have had the opportunity to spill this information, what would Michael care. The message to Roth would be that Michael had made plans to avoid the planned hit. Your comment about the troops coming to guard Roth are interesting, but I think it is just as likely that Batista, who knew by then his time was short, got work that Roth was in the hospital, and he simplky sent his people there to protect him.... and likely to get him out of Cuba.