MeSo, I'm only going to address one thing okay, because really, it's virtually the only way any of us can stay upright with all we deal with. I'm ver sad about the composite picture and I also know how everything feels interconnected, but it won't get resolved that way. I REALLY get it. Okay about the pagan ceremony, here's a suggestion that may not work at all, but even if it doesn't maybe it could morph into an idea that will work. Here goes:
1) Is there someone who could ask either your mother or your father, in a very nice, social way, just what will be included in the ceremony. Possibly, it won't be as difficult as you are fearing it might be -and I'm not denying that it could be - but forewarned is forearmed.
2) If you know ahead of time, as it seems you are quite decided about going, you can try to mentally prepare yourself and enlist the help of your husband, your therapist, a friend - whoever might do that safely
3) you could perhaps enlist someone else to research pagan ceremonies on-line or at a bookstore, if you have some idea what your parents are into

Maybe none of the above is possible or feels safe, but let it simmer and perhaps it will give you some other ideas. I can tell that you are a very intelligent person and I think you can find your way through this.
I faced a difficult time when my daughter married into a faith where I was not allowed to attend the ceremony and I'm not going to expalin that so as not to offend. Sometimes couples will opt to have two ceremonies to accommodate both sides of the family - they refused. They also insisted on having my side of the family present for the reception - my mother and 6 brothers whom i had "divorced" a few years prior for the same reasons you had to. i had to face my mother at my daughter's wedding. My best friend stuck beside me, glued to me and this is a situation where dissociation is still a pretty good coping mechanism, temporarily. Perhaps there is a part of you able to cope in this situation and even if that part is unknown to you at this time, i have come to trust the they show up as needed, when needed, just like they did when very bad things happened. If the reminder is prresent, likely, they will be too.
I hope this helps, if nothing else, to know that you have been heard and that your struggles have been honoured, as well as your enduring courage and strength. don't forget to breathe. if safe