When I first read your post I could not see the forest for the trees.
The AI has an annoying built-in cheat that allows bribes to occur, even without using a Dip... and it is not a scenario-related thing. I call it the "Contact Bribe" because the way the AI does it.
Here are some of the details, according to my own observations (but not parametrically tested):
1. Any AI combat unit can "Contact Bribe" human units.
2. No gold is deducte from the AI treasury.
3. The bribes can occur even if the human's units are democratic (yes, it's true)
4. The bribe probability is a psuedo-random, and increased if the human unit is blocking a choke point.
5. Ships and aircraft and cities are not bribed this way.
6. The bribe occurs in the instant the AI unit bumps into or begins its move next to the human unit.
7. AI dips and spies are not necessary to do the human the dirty.
8. AI dips and spies cannot revolt or subvert a human democratic city.
9. Bribes by AI dips and spies cost the AI money.
10. When a "contact bribe" occurs, your former unit switches sides, and often attacks the first Human unit it can find.
