Geary setting up Michael is possible but highly debatable.

The assumption that Geary knew that the FBI was holding Pentangeli for a perjury trap is somewhat tenuous. It is not likely that such a secret, high-stakes operation would be shared with all the members of the subcommittee, especially when the intended victim clearly has political connections.

Furthermore, it is likely that Roth knew that Geary was in Michael's pocket, especially if you believe Fredo was feeding Roth information (which I don't, but many on the boards do). In either case, Roth certainly knew about Geary's sudden about face regarding the casino license and probably figured out what was up. Via Questadt, he would have ensured that Geary was out of the loop regarding Pentangeli.

That would have been a safer course of action than trying to convince Geary to switch sides. Even if convicted, Michael would have been able to arrange the destruction of Geary's career, if not a murder prosecution.