I’d like to go through the reasons why Roth didn’t engineer Frankie’s survival one more time:

First, it would have been nearly impossible for Roth to arrange the cop’s entry to save Frankie when literally a few seconds separated him from life and death. The cop was not part of the plan. He was surprised that Richie’s bar was open, and immediately suspicious and wary of Frankie’s legs sticking out of the doorway to the rear of the bar. The Rosatos weren’t expecting the cop. They panicked when he started toward Frankie’s body. Carmine Rosato almost blew his head off (until Richie cried, “Carmine, NO-O-O-O…”). They dashed out of the bar, knocking the cop over, and ran into a gunfight. The cop was wounded in the gunfight—you see his partner propping him up near their patrol car. No way was that cop part of the plan.
If he wasn’t, could he have been an unwitting patsy for Roth? If so, how? Did some higher-up in the police department who was on Roth’s payroll call the cop and tell him, “OK, Shultz, show up at Richie’s bar at exactly 3:42 and thirty six seconds on Tuesday afternoon—ya got that? Three forty two and thirty six seconds. Now let’s synchronize watches…” Did Roth or someone in his employ call the precinct and tell them, “Something fishy’s going on in Richie’s bar; I think you should send someone, but not until three forty two and thirty six seconds”; or hope that the cop would arrive at the exact moment to save Frankie? I don’t think so.

Second, Frankie had lured Michael to Havana for two purposes: to get the $2 million, and to have him whacked by his government pals. Why would he risk everything on an impossible, split-second-timed plan to save Frankie so that he could testify against Michael months down the road—when, according to Roth’s plan, Michael would already be long dead?
90caliber offers an interesting idea: Frankie’s survival was a backup for Roth in case the Havana assassination failed. Since the Tahoe assassination failed, Roth might have wanted a backup for Havana. But I don’t think so:
The Tahoe plan was far riskier than Havana. Roth needed to sneak two gunmen out of New York, and their machineguns, into Michael’s heavily guarded compound, then arrange for them to be killed. Very risky. But Roth already had a contingency: Frank Pentangeli. He knew that Frankie would be at the party, contentious over Michael’s support of Roth and the Rosatos. Therefore, Frankie would make the perfect fall guy. So, if the hit succeeded, Michael’s successors would come after Frankie. And, even if it failed, Frankie would still be regarded as the guilty party—and Michael would come after him. That’s exactly what Michael led him to believe when he visited Roth in Miami.
By contrast, the Havana assassination plan was a slam-dunk for Roth. He was the most powerful gringo in Havana, thanks to his long-term friendship and partnership with the Cuban President. He could have his pal Batista squash Michael like a bug. What was Michael’s bodyguard (formidable though he seemed) compared with all the military that Roth could bring to bear? We saw what happened when that elite-looking military group double-timed down the hospital corridor and riddled the bodyguard with bullets without breaking stride. Roth didn’t think he needed a backup because, as far as he was concerned, Michael was a dead man the moment he stepped off the plane in Havana. Too bad for Roth that he never counted on Michael figuring out that he was behind the Tahoe shooting; and Batista abdicating.

So, why did Tony Rosato say, “Michael Corleone says hello”? As others have pointed out, the script went through many iterations; and in an earlier iteration, the script called for Roth having engineered Frankie’s survival. Also, as others have noted, GFII, like all movies, was filmed in sequences that suited the production schedule, not that followed the actual running narrative. A simple explanation is that the “Michael Corleone says hello” scene was filmed while the Roth-engineering-Frankie’s-survival idea was part of the script. Later, FFC thought better of that idea (probably because it was too far-fetched) and dropped it, but retained the earlier filmed sequence—either because he liked the way it played, or because it was already in the can and some or all of the actors had been released, making it too hard (or costly) to reshoot, or because he was careless.
This happened elsewhere in GFII:
Last year, one of our sharp-eyed posters spotted Questadt in the businessmen’s meeting with Batista—sitting in a chair right behind Roth. There was absolutely no logical reason for Questadt to be in that scene—none whatsoever. What’s more, if Michael had noticed him there, he’d have known he “belonged to Roth”; and when Questadt turned up as the Senate lawyer, Michael would never, ever have perjured himself. So what happened? Sure enough, another poster turned up an earlier version of the script in which Tom and Michael refer to Questadt before the Havana meeting as someone who could help them with Geary and gaming matters in Nevada. Evidently the businessmen’s meeting scene was filmed while that script was still operative. Later, FFC reconsidered and dropped that idea, reserving Questadt solely as the Senate lawyer. But he retained the scene with Questadt sitting behind Roth—probably because he didn’t want to have to reshoot it, and figured that nobody’d notice Questadt in it. Of course, he hadn’t counted on the sharp-eyed people on these Boards!