Knowing about the Geary-Michael connection would have made it very risky for Roth to tell Geary about Pentangeli.

While dt's scenario is plausible, I think it would be equally likely that Geary would have used that knowledge to barter a truce with Michael, leaving Roth with no weapon against Michael.

Also, Fredo knowing about Questadt does not necessarily mean that Fredo continued a relationship with Roth. He might have met Questadt through Roth or Ola before the Tahoe hit.