Regardless of what Danny Aiello says, I cannot believe that the "Michael Corleone says hello" line is an ad-lib.
This was one of Danny Aiello's first roles, and he's working for a celebrated director on the sequel to a Best Picture winner. The thought that he'd ad-lib a line during a pivotal scene - in such a way that we're still discussing it 30+ years later - strains credulity.

On the other hand, there's no other explanation for the line. Even if you accept that the police officer's entrance was planned to set up the perjury trap -which I don't - why include that line. After all, Michael ordered Frankie to meet with the Rosato brothers. That fact alone could lead Frankie to believe that Michael betrayed him.

As for the bartender, the Rosatos planned a hit to occur right in front of him. They obviously felt he would keep his mouth shut, with or without feeding him a gratuitous line (which would hardly diminish their legal culpability).

As Apple says, it's a flaw.