Regardless if gov type, a civ's capitol can never be incited to revolt or bribed.
I can verify that you cannot bribe an AI capital, I have tried it many times and it never works. You must "take" a capital by military means.
Spies cannot "take out" the palace, at least, not in MPG! There is a thread about that somewhere and it looked so promising I just had to try it. Sorry, no can do. Your palace cannot be destroyed or sold. The only way to get rid of your palace is to build a new one in a different city. (Yes, I do miss the feature in civ 1 were you could sell your palace. It came in handy occasionally went you just had to have a couple of coins to save the kingdom!!)