I haven't even read your post thoroughly, but I do have this comment. Start you letter off this way:
Dear Mom and Dad,
-"well, you can't visit us unless you're in the truth ha ha ha"-"we don't want any apostates in this house ho ho ho".
"Mom, Dad, I have to tell you just how completely sick to my very soul that kind of talk makes me. Even if you are only sort of joking to "encourage me", being raised as a Witness, I know just how close to home those words hit in some dysfunctional family members minds. This sort of emotional blackmail is disgusting to me, especially now that I am a parent (use it if you are, Expat).
I know you guys only want the best for me, but what is best for me is up to me to decide at this point. Just like every other human on this planet....."
Anyway, you know your parents, but I really do advocate a bit harder hitting methodology than most people I have seen on these forums have taken. Remember, WE (apostates) are the ones who have at least a desire for truth and a passion for honesty on our sides. Use it.
Cautiously, of course.