"Who killed the Tahoe shooters" is the Number-One unanswered question on these boards. Some have theories, but none are definitive.

As for why Michael left Tahoe suddenly:

Michael knew that, although he was the target, his enemy had no scruples about killing Kay or anyone who got in the way at Tahoe. So, by removing himself from Tahoe, he removed the target. More important: by sucking up to Roth, he bought precious time to protect himself and his family. His guiding principle, as he told Hagen before he left, was: think as those around you think.

Michael saw that Roth was very careful to mask his role in the Tahoe attack. Roth waited for Anthony’s party because he knew that Pentangeli would be there, contentious over Michael’s support of the Rosato Brothers. Frankie therefore had perfect motivation to kill Michael, and would have made a perfect patsy. Since the attack failed, Michael figured that Roth would have to strike again quickly, before Michael figured out that Pentangeli wasn’t the guilty party. By visiting Roth in Miami, by affirming that their deal was still good, and most important, by agreeing to go to Havana, Michael was helping Roth to find a reason to keep him alive until then, and to leave his family alone.

Michael alive in Havana would provide Roth with an airtight alibi: Roth would stage the father/son routine at the birthday party, and turn over his empire to Michael in front of many witnesses, so he’d look like he had no reason whatsoever to kill Michael. And as the most powerful gringo in Havana, Roth could easily arrange to have his pals in the Cuban government assassinate Michael and get rid of his body. If anyone asked after the missing Michael, Roth could speculate that he’d been caught in the crossfire between the government and the rebels. Or that rival gangsters whacked him because they were jealous that Roth planned to give him nearly all of the Havana empire. The only thing Roth overlooked was that Michael was onto his game and, with “Sicilian cunning,” outwitted him.
Even Michael’s choice of bodyguard was designed to mislead Roth. He didn’t take Neri and/or Rocco with him because, at that point, he couldn’t be sure of their loyalty. And, by bringing Buscetta to Havana, Michael was signaling Roth that he still didn’t know who the traitor was—but it obviously wasn’t Roth because Michael was there with Roth. He had accomplished what he told Frankie he’d do: Roth was relaxed, confident that their deal was still good—and off his guard. Bravo, Michael!