It's clear that the bartender knew the Rosatos. That doesn't mean that the bar was in Rosato territory. I think it's unlikely Frankie would have gone into the lion's den for that meeting. He hardly trusted the Rosatos.

I've never agreed with TB's theory of the "Michael Corleone says hello" line. They never would have committed a murder in front of someone they thought would give the police any information whatsoever. If they thought the bartender would testify honestly, they'd hardly be safe with him telling the police, "The killer was named Carmine and looked like..., and he said 'Michael Corleone says hello.'"

In any case, I doubt they intended that Frankie's body would be found at the bar, so there would have been nothing to connect Richie to the killing.

That line must be either a vestige of an earlier script or an unusually clumsy device to trigger Frankie's betrayal.