It's not bribery, so the cash infusion to the rebels should make no difference. There's been some discussion on this, and the consensus was that you should be able to limit the effect by grouping FMs and/or adding agents (diplomat/spy/assassin) to the stack. I'm very surprised to hear that you are doing this and STILL seeing defections. Nobody else has reported that.

To adjust revolt frequency for FMs:

Quote Originally Posted by QuintusSertorius View Post
It's random for low-Loyalty characters. I must admit in all the time I've played the game, I've never had it happen to me.

You'd have to edit the descr_campaign_db, under <revolt> change this:

Code:
<max_revolt_chance float = "50.0"/>
As usual, not savegame compatible.
For captain-led armies its:

Quote Originally Posted by QuintusSertorius View Post
Uh, that's for FMs, I thought you were talking about them. Captain revolts are a different one:

Code:
<captain_modifier float = "0.01"/>
It's already a tiny proportion. I'm not sure what happens if you make it 0.00 that might revert to the default setting for vanilla, so you might be better off making it 0.000000000001 or something.