I wouldn't worry so much about the Rosato brother (Danny Aiello) saying "Michael Corleone says, Hello".

First of all, that saying was simply an ad-lib by Danny Aiello, nothing more, nothing less. That line was never in the script. But Coppola left it in because, very often, when the mob kills somebody, they want the victim to know who killed them so it will be the last thing on their (the victim's) mind before they die.

Roth wanted Frankie killed because, once Frankie was out of the way, that would pave the way for the Rosato's (&,in turn, Roth) to take over the Corleone Family in New York.

Also, don't discount the Senate hearings. Remember, Sen. Questadt was on the panel questioning Michael. He was sitting behind Roth at the Havana meeting. Remember Fredo telling Michael: "The senate lawyer, Questadt, he belongs to Roth". So, it's safe to assume that, since Roth couldn't kill Michael in Tahoe OR Havana (remember, NOBODY expected the rebels to take over that night), Roth figured that he would have Questadt form a senate subcommitee to investigate Michael & get him on a charge of perjury. The fact that Frankie lived was a bonus for the senate (& Roth) because then, thinking Michael turned on him, he could be used as a star witness against Michael.