90caliber: I acknowledge where you see an apparent contradiction. I should have been clearer:
One of the reasons that Roth would not deliberately engineer Frankie's survival was that Frankie's testimony could involve him and expose his criminal activities. But after that fateful New Year's Eve, Roth was in desperate shape: he'd suffered a stroke, lost his Cuban gaming empire, and learned the hard way that Michael knew all along that he was behind the Tahoe shooting, and had found out Fredo's treachery. But Roth, crafty fellow, also saw that Frankie's survival provided him with an opportunity to trap Michael through his puppet, Questadt. Yes, if Frankie had testified against Michael, he might well have harmed Roth. But at that point, Roth had nothing left to use against Michael.
You might ask: Why would he still want to go after Michael when he'd lost everything, and using Frankie could expose him? As we saw constantly in the Trilogy: the desire for revenge is most often emotional, not rational. But Roth also may have believed that if Michael went to prison on perjury charges, he'd have a chance, down the road, to pick up some of the pieces of the Corleone empire. The guy was nothing if not resilient.