Op next time you discuss this with your son, it might be worth changing the discussion around a bit before jumping straight in with ‘please do not make me sit next to my ex’ - because as some of the answers on this thread have shown, they don’t listen or think about the situation from your point of view and just default to ‘suck it up for a few hours’, which is what it sounds like your ds and dil are thinking - they have seen it work at other weddings where the exes get on amicably and think that’s what they want, and that what they are going to get. They may even naively think that they are helping you, showing you that with time gone past, your ex is not so bad and that you can sit together quite happily and that they will fix it now so that you can both be at all events going forward... who knows? But it might explain why they seem so set on this and creating their happy family dream...
Instead, you need to start the conversation back with what your ds remembers about when his dad lived with you all. And why does he think you split up from him? And why does he think that he didn’t didn’t see a lot of his father growing up?
From what you have said and a bit of extrapolating on my part, chances are that he has normalised it to just being you both arguing, both being unhappy, and his dad being useless with money and younger kids so stayed out of the picture... in which case you have done amazingly well in ensuring your ds has not been affected but unfortunately that has now led to these problems.
I would then change topic to domestic violence/abuse, coercive control and financial abuse in a marriage, and what they understand it to be, different levels, difficulties in getting away, when do you leave, what level is acceptable (I know, their immediate reaction should be it’s not acceptable at all, but you need to get them to make this judgement themselves rather than telling them so that it is their own brain that is working this out rather than just being passive listeners if that makes sense). List out a whole load of things from being slapped and punched, through the nightmare of things that he did to you and on to ‘accidental’ attempted murder (which category I would put the hair fire and iron branding into as a rational person knows they are horrendous dangerous terrible things to do to people but I guess an abuser would try to minimise), actual attempted murder (strangling, hitting, punching, kicking etc repeatedly that are so bad the victim is lucky to be alive and that the abuser can’t wiggle out of with an ‘oops my foot accidentally slipped and kicked them 23 times’) and of course ending up in murder.
Then talk about survivors of this sort of abuse, how it can affect the rest of their lives, PTSD, how bad does the abuse have to be or have gone on for for people to suffer through time as a result, hopefully they will again work out the answer for themselves that even low level abuse can affect people for life. Don’t mention what your ex did yet - you want them to be thinking that low level abuse is bad.
Then on to is it reasonable to expect the victim to have to deal with their abuser again - and if so - when? Where? How? Time elapsed? Proximity? and so on... Again hopefully they will say no contact or will at least realise that for even low level abuse the victim shouldn’t have to come into close contact with their abuser and if they do end up at the same event they should be kept apart.
Only once they have fully comprehended and understood the general situation, that’s when you can finally say that the reason you wanted to discuss this with them is because you wanted to let them know that in the past you wanted to protect ds as he was small but now he needs to know why you no longer have anything to do with your ex - and that while ds might think of him as dad, you can only think of him as your abuser, and explain what he did and how it has left you.
From here on in, try to make sure that you only ever call him your abuser (or psychopathic nightmare or similar) never call him his name or ‘your father/dad’ or your ex - don’t give him the dignity of a name that sounds normal that they can like and use to normalise him again.
Hopefully it now won’t you need to spell it out to them that just moments before they had agreed that victims of even low level abuse should have minimal if any contact with their abusers, as you are the victim of severe domestic abuse and violence, then surely they can understand why you don’t want to sit next to your abuser. And that obviously while you would prefer never to see him again you understand that he is ds’s father so if he does want him there then all you are asking is that you don’t have to sit next to him but next to dh, who has been your rock over the last 20 years.
Hopefully this way they will understand and not want your abuser there at all but as a minimum will keep you well apart