Not entirely implausible. But also a major stretch, even by the standards of this board. I don't see any evidence in the film to support this, especially since Fredo is later revealed to be Roth's inside man.

Furthermore, Michael seemed to view killing Roth as a matter of great importance. I don't see why he would send someone who he considered a possible traitor on a high-priority suicide mission. That kind of assignment would seem to be best carried out by your most trusted operative, not your least trusted.