Hi OP, I've read your posts and skim-read the thread, so apologies if I'm saying the same as someone else.
Most people at our school did whole-class parties in the Reception year, to help the kids get to know each other (and the parents too), so there will probably be quite a few more for your DD.
I kept a box of generic gifts, a few PlayMobil boxed figures, £5 Lego sets etc, with a stack of appropriate birthday cards, and gift bags.
As time went on and friendship groups formed, subsequent birthdays were much smaller, with 5-10 friends invited. By then we knew the children, so found it easier to choose a gift, but generic ones were still fine. Sometimes I'd ask the parents what the child liked when RSVPing to the party, to get a guide for a present.
By 8/9 some just have a day out/activity with a few friends, while others still do a whole class thing.
The big parties seem to start up again when they reach their teens and have multiple social groups, but by then the kids are leading the plans so you don't have to figure it out for them.
My 11yo is autistic & adhd, and I needed to 'hover' like you when he was smaller. Many people are oblivious to others' differences, so it can be helpful to explain a little, as they often misinterpreted his behaviour.
It's worth making the effort to attend if your DD wants to/or is ambivalent - if you don't go a few times they may stop inviting her, which can lead to difficulties at school as those left out of one activity are often left out of others too.
Maybe your DH can take her to the next one!
My DS found parties tricky and didn't join in, so the invitations dwindled away and then stopped. He hasn't been to any birthday party for a few years, but likewise only has a couple of friends to his own (and on their birthdays they did activities he didn't like).
It's a fine balance between feeling left out and accepting he didn't actually like going anyway. His brother loves the social aspect, but can't cope with the noise, so finds parties pretty difficult too.
It sounds to me like you handled the party fine, and now you know about the present expectation, it should be smooth sailing next time. Fingers crossed the overbearing mum keeps her rude and unnecessary opinions to herself next time! 