Yes, all I can suggest is you talk to them each separately, stating this is the single most important day of my life, and it's important that you BOTH come, and that you both put aside your differences for the few hours it will take. Let them know the other one will get the same offer, and if only one can agree, then only they are invited, and that on the day, should they feel they really can't control their feelings they should leave, not ruin your day.
I'm not sure you should threaten them with being written out of your future, however, at least try to appeal to their parental instincts first.
A ray of optimism for you, don't be too concerned about the imbalance of her family and yours, the moment you say 'I do' they ARE your family.
In the end, do what you need to do to, so your day goes well, even if it means doing it without them there.
As for the seating arrangement I don't think it's law to split em like that, just get some over on your side to balance the numbers or it'll look weird in the church

oh and invite friends! That should build up your numbers some.
Congratulations on finding someone special however, and I hope it all goes well and that your parents see sense, and can moderate themselves for the sake of their son's big day.
Oh last thing, for pity's sake, let them know it's just them, invited, not any new partners, at least that's my suggestion, I can't see this being the ideal place for those two to meet new partners of their exes.