I have no idea to the first question.
I am not sure that DVIP modifies the chances at all, but since I have switched to playing it the retribution if you are discovered has increased considerably. I have checked neither release notes nor game files so I might be mistaken, but what seems to happen in vanilla DV if you attempt to assassinate someone and are discovered, the ruler of the other court would make one attempt to assassinate a random courtier in your court. In DVIP what seems to happen is that the assissination attempts continue until one succeeds, and that members of your family are targetted.
I don't think I've tried assassinating anyone in my own court since playing DVIP so I don't know what happens there. In fact after a few assassination atttempts of mine I have stopped using it as a mechanism as the consequences of being discovered are too damaging. I am not sure the mechanism in DVIP is correct, but certainly assassination of foreign heirs in vanilla DV is too easy.