TB, I'm with you 100% up until the "breathes deeply" paragraph. The timing involved in such a scenario is tough for me to get my arms around. Another thing that I might consider, though, is that Fredo, if noticed in the area of the gunmen, could simply have said that he was out for an evening walk, and seeing the gunmen immediately knew that a hit on Michael was on. So he took action by killing them both, and would be viewed as a hero and savior, thereby garnering a tremendous amount of trust to use in taking Michael out at a later time.