Forgive me if I'm wrong, but you have stated in the past that you have greatly improved the sitch in the past with positive behaviors, and antagonism seems to only escalate things.
However, in your head, you keep assuming the worst, whether its a Christmas card from BIL to X's contacts with you. Why not try acting 'As-If' and see what happens? This doesn't mean disrespecting yourself while he's in an R with OW2, but truly shifting your train of thought so that you reflect a more positive attitude.
With a paranoid guy like Andy, he's likely to be very good at noticing threat, and your anger/resentment of him is not likely to yield positive behaviors from him.
You can limit set, while being positive. You're not some sort of behavioral therapist for Andy, working on altering his behavior via reward or punishment. You are his ex-spouse, targetting the relationship b/t the two of you, altering the piece of it that you are in control of - you. Whether this benefits your R with Andy or not, it will help you with other and future R's regardless.
IMHO, I say shift your attitude and work to interact pleasantly with the BIL, keeping things light and playful with Andy. You can do this while keeping appropriate boundaries up.
