You see. Have you not see there isn't any diplo required here. We took your gold, fine take it back, maybe some science if you fancy that. No big deal.
That's exactly what another kd took back when we took their gold. They made 26 learns vs 3 plunders. We offered CF, they accepted it. End of story. No diplo required for mature kingdoms.
Moral of the story: Do not start back the PK culture.
Edit: If PK started to go around, kingdoms will resort it to take out the top kingdoms, just to prevent them to contending whatever they wanted to contend.