TB, that's why I say that FFC had to have a way to let the audience know that Michael planned to murder Roth. That the reason for that incongruous conversation between Michael and Fredo. That conversation also further paints Fredo as a vulnerable, naive, and insecure sibling that will prey on the audience's sympathy and make the fratricide all the more horrible.

But regarding the soldiers showing up at the hospital: I still think it was on Batista's orders to secure Roth's exit from the country. I don't believe that if further betrayal was in Fredo's heart that he would have waited several hours to exercise it.