With all of the wedding planning and guides out there, and how elaborate this wedding sounds, I am a little shocked that the bride-to-be thinks it's acceptable to have open seating for that many guests. I had 100 people at my wedding last August and would not have dreamed of letting it be open seating (my DH and I each have some small family tiffs on either side due to some tense relationships so I wanted to be extra mindful of that). Every single one of my guests was from out of town (we moved to Boston a week before the pandemic shut everything down), and our family and friends were coming in from all corners of the U.S. except for Boston. I would have loved to have a cocktail reception for everyone after the rehearsal dinner, but with a large wedding party (care of my DH) we already had 50 people at the rehearsal inclusive of party (18), their wedded or engaged partners, and immediate family. We had to make cuts. It helped that my DH and I paid for the vast majority of our wedding, so we (I) felt comfortable calling the shots and putting my foot down.
OP, I am so very sorry for the feelings you are feeling leading up to this. Money makes people a little extra cuckoo, and weddings even more so. From the details you've provided I believe you and your family have gone above and beyond in your commitments to your son for this occasion. I hope that on the days of the events you can know that you did what was within your control and you can let loose and have a little fun. For the parent-child dances, maybe you can suggest the dance battle leads into a combined father-daughter and mother-son slow dance, so you can at least get that time together. My DH and I did that because he didn't want parent dances at all and I knew my dad would be disappointed if we didn't do it, so I picked a song and lo and behold my MIL had it on her list as well.