I think the threat would have to be against revealing the Brotherhood, which she hasn't necessarily been trying to do, and not just against outing one of it's members. As Westie has pointed out, though, Davina's family is immensely powerful, so any move against her requires a considerable amount of caution on the Earl's part.
As far as Kerry knowing so much about what was happening with Bridgemont, a certain amount of that must have been public knowledge. Even today, we have the Fortune/Forbes lists of wealthy people. Beyond that, i would think that Caroline would have shared much with him, knowing that he was going to meet with George, and knowing that she could trust him. The big question is how did she know she could trust him?