This type of thing is exactly why dh and I didn't have a wedding. I'm not even exaggerating. Theres ni way we could have done it without someone having a tantrum and my parents and step parents have caused so much anxiety, stress and upset with their inability to realise not all family occasions are about them and their feelings about each other.
Every single family event where they both have to go has been spoiled our clouded by a million types of micro managing we've had to do to manage supposedly grown adults emotions to make sure they aren't giving filthy looks, getting drunk or as often happened, arguing and moaning to other guests about each other.
My brother's wedding was spoiled by my stepmum demanding where everyone sat.
Dad thought his wife (our stepmum) should be on the top table (in place of our mum if they got her way) and said it would be extremely disrespectful if she wasn't.
But stepmum also didn't want to be sat anywhere near my mother and wanted her with sils family on the end. She ideally felt like Mum shouldn't be on the top table because Mam cheated on my father over 20 years ago. (She forgets that dad cheated on mum and beat her up) My mum didn't want to be anywhere near my dad (because of his abuse) and felt like because she raised us she got more say and she'd have been hurt to be plonked at the other end.
My brother ended up not having a top table at all, sil was also called a bridezilla for making that choice by stepmum and instead of being able to enjoy their wedding day, they had our parents moaning and moaning the whole time.
It's not actually sil and bil who cared about appearances. It was our parents, stepmum and Dad mostly but also our mum. The emotional blackmail and bitching and "suggestions" about how things should be done took all joy out of my brothers day, and many many other special occasions.
Personally I wouldn't make any suggestions, if the level of dysfunction really is to a degree your dh and his ex can't leave their drama and hatred for each other at home for the sake of their son, then maybe neither of them should be at the top table.