And one more thing I just thought of: If the Tahoe assassins were "out of New York", they were likely Rosato buttonmen. IF that's the case (and I'm not saying it is) then the Rosatos killing Pentangeli would only serve as a reward for their incompetence in Tahoe.
Roth was behind the attempt and would likely used his own assassins. Probably old members of Murder Inc.
The Pentangeli-Rosato conflict was a local matter and did not involve Michael directly. In fact, Michael supported their claim to those territories in the Bronx and ordered Pentangeli to give it to them. The Rosatos were just pawns in Roth's scheme. They wanted Pentangeli out of the way, and this would weaken Michael's position, which is what Roth intended.
