As for the Danny Aiello line, I do not believe that it was an ad-lib or a message to the bartender. I think it is merely the remnant of a discarded plot line.
I believe someone here surfaced an earlier script in which Roth did plan to have Frankie survive--and the line was intended to set Frankie against Michael. That plotline was dropped for obvious reasons--much too far-fetched. But the "Michael Corleone says hello" line was far too prominent to have been left in by mistake. That's why I believe FFC rationalized it as something for Ritchie the bartender.
