Thanks for your courtesy, LW. It gets a little dodgy, but I'll try:
Sure, it seems contradictory that Michael first tells Roth, "Frank Pentangeli is a dead man," and then dispatches him to "settle these problems with the Rosato brothers." If the first statement was designed to assure Roth that Michael was convinced that Frankie was the perp in the Tahoe shooting, then the second would have alarmed Roth.

But, I think it can become clearer if we keep in mind that Michael's utmost priority at that point was to buy time in order to find out who the traitor was in his family. He visited Roth before he dispatched Frankie, and fell over backward to convince Roth that the big picture was that "nothing interfere with our deal." The $2 million was the key to the deal. I think that Michael figured that not killing Frankie would make Roth suspicious--but Roth's greed for the $2 million would overcome any temptation for Roth to whack Michael right away. Michael figured, correctly, that Roth would wait until Michael came to Cuba with the $2 million. Once he got the money, he could have his pals in the Cuban government squash Michael like a bug.

Meanwhile, by sending Frankie to the Rosatos, Michael had himself a win/win: if the troubles were settled, it'd be one less thing for him to worry about. If the Rosatos moved against Frankie, it'd be one more piece of evidence that Roth was behind the Tahoe shooting.
Calculated risks, all, but it was Michael at his most Sicilian-cunning.