The "greeting" supposedly from Michael always (and still) confuses me. I first assumed that Mike really was behind it given (as noted) the unforeseen incident of the cop walking in (I did not and do not think that was planned but who knows?). The rest of the context, though, indicates that Mike really did not want to have Frank killed (at least at that time). On the other hand, maybe it was to show just how devious Mike could be (even to the audience). I always thought, given Frank's brother's facial expressions, that he was not a prisoner, per se, but there to remind Frank of his vows and the consequenses of breaking those vows. By the way, sort of the reverse setup between Frank and his brother and Mike and Fredo in that unlike Fredo, Frank's brother kept to the code and unlike Mike, Frank (perhaps) acted to save his brother