Brave does affect the percentage chance of a reaction ability working (subtract Brave from 100, that's the chance that a reaction skill will NOT be used. So if you have 97 Brave there's a 3% chance that the reaction ability will not work).

Not sure about the Geomancer thing, I'll have to check it out.

Also, Ramza can safely have any Brave and Faith, and he will not leave the party. So you can make him at 03 Brave and 97 Faith, for example, but he'll still stay.