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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.
 
In DVIP, from my experience they kill one of your heir when you kill someone of their court

It is more likely to succeed with a high combination of intrigue from you and your spymaster, and low stats on the other side

Courtier are easy to kill and king are unlikely to be assassinated
 
I don't know any specifics but I do know that your rulers intrigue, including the spy master, count towards your success. Of course, the intrigue of ruler (and their spy master) of the realm the assassination is taking place in can counter it. I THINK the target's intrigue matters too. Basically, have a really good intrigue and spymaster, and hope your target's lord has a really crappy one.