if Fredo is "in" on alot of what Roth is up to (he knows about Questadt), then why not reveal to Roth the frameup of Geary? Certainly Roth could use that knowledge to his advantage possibly even neutralizing Mike's use of it.
Maybe the reason Roth decided to put Questadt in his pocket is because of Geary.
I think the movie is beginning to make more sense if you put together everything that Fredo could have told Roth and correlate that to what Roth's actions are.