cricket, with all this conflict and chaos and the fact that the AH and family work counter to whatever you plan, it would be best to STOP trying to plan things, especially big things like hosting 20+ in your home. you might as well plan to have it in the middle of the freeway at rush hour.
you are in a lousy situation, with an ah who works against you at every turn, and who uses the children against you as well. really sucks for the kids...they have no choice and are going to align with the easier "whatever" parent.
it's just wrong that you are left to DO "everything" while everybody else bails. but that's what you have on your hands. the only way to not feel abused is to not put yourself in that scenario. it's probably time to consider more drastic measures.....