there is domestic abuse, coercive control and a history of manipulative and verbal abuse towards both children from Dad
There is no court order in place.
Hasn't seen SEND for over a year. NT sporadically
She's not attended for contact in over a year, due to inflexible rules. Added to that messages since calling her ungrateful and rude
there has been a severe breakdown in the relationship for both children.
Having read all that, it's a no brainer for me. Of course you can't allow one child to attend and not the other, you're mad to entertain the idea. His inflexibility is appalling. Given that he's only had sporadic contact with your NT child, it's completely inappropriate for them to have a prolonged visit over the time of the wedding, anyway, even if ND child was going.
Tell the children that neither are going and it's not up for debate. Plan something really exciting - preferably something that gives you an excuse to buy them both a special outfit to wear (if that's what DC2 was excited about). DC2 will be disappointed but you have to tell them that it's wrong to exclude people - and excluding disabled people is discriminatory - which is why she can't go. You must be very clear that it is NOT DC1's fault that DC2 can't go, it is their father's fault for excluding them and for refusing all the possible solutions you've suggested.
Also, I don't see why NT child should have to see him at all. There is no court order so you won't be in breach of anything. It sounds as if contact with him is not in her best interests.
Do not allow him to continue to abuse your children in this way.