Geary's embarrassingly condescending rant about Italian Americans,at first glance, would indicate that he was still under Mike's thumb because of the hooker incident at Fredo's place. As we find out,Fredo was more involved with Roth than originally thought,i.e.,he knew about the Senate lawyer being "Roth's man".
Could Fredo have tipped Roth to the fact that Geary was compromised? Roth then would have had two ways to go.
1)Instruct Questadt to keep Pentangeli's existence secret from Geary until the last minute to let Geary (unintentionally) bury Mike.Seems unlikely.
2) Get the word to Geary that he (Roth) knew about the hooker (from Fredo),and that this was a chance for both of them to put Mike away,and eliminate any credible evidence of the hooker murder for good.Seems feasible
Personally,I don't think that Geary,or any member of the Committee,was unaware of the fact that Pentangeli was going to appear. Don't forget the scene where the Chairman goes ballistic about the affadavit from Pentangeli that they had prior to the hearing.So Geary puts on the show for Mike,ducks out,figuring that Five Angels will take care of his "Corleone problem". Geary was stupid,Mike was lucky.

Either way,I don't think any conclusion can be drawn from the fact that Geary wasn't killed later on. After all he was a Senator,and the heat would have been incredible.