Good questions, Louren.
Michael left Tahoe immediately for his family's safety. He was almost certain that Roth was behind the failed attack. Roth was ruthless enough to have his bedroom machine-gunned, even if that meant that Kay and/or the kids would be killed, too. And, because Roth had someone inside the compound helping him, he'd try again, just as ruthlessly. But, as long as Roth believed that Michael didn't suspect him--and, more important, that he could get the $2 million from him--Michael would be able to buy time to figure out who the traitor was.

I think that Michael could have flown via commuter plane from Tahoe to Vegas, where he could have gotten a commercial airliner to Miami. Going by train, IMO, was directorial license--time to give us a good look at the bodyguard, and to see Michael swallowing a pill (I bet it was Miltown, the tranquilizer of choice in the late '50's). Flying would have been safer than traveling by train.

Yes, ordinarily gangsters in fear for their lives wouldn't let their plans be known. But, since Michael was sure Roth wouldn't try again until he got the $2 million, it'd be in Michael's interest to let Roth know he was coming--that way, Roth would be relaxed, thinking that their deal was still on, and that Michael suspected Pentangeli, which is just how Roth set it up in Tahoe.

As for the plane in Cuba: My guess is that as soon as Michael figured that Roth was going to have him assassinated on his way back from the Presidential New Year's Eve party, he called Tom and had him arrange the charter plane to be standing by. Or, Michael could have arranged it himself. Not a big deal.