I understand where you're coming from, Heidi.
It sounds as if your DSD could be under quite a bit of pressure, self-imposed or otherwise, about special occasions. I am sure this will change as she gets older as it sounds as if you have a great relationship and it was a big thing that she explained the 'mum would be angry if we open presents with you' even if it doesn't change the outcome just yet.
I went to DSD's birthday parties for the first 3-4 years. They were/are organised and funded by DH and his ex jointly.
I mucked in, doing messy/boring jobs beforehand and helping to clear up afterwards. Helped if needed during, such as topping up drinks etc but I stayed on the sidelines a little for key moments such as the cake being brought out/candles blown out etc as I totally understand that it is a big thing for parents.
Then for the last one, out of the blue, the ex said I was not invited. This despite no problems between us and me not even having seen her since the last one. She told DH she wanted DSD's family to be seen as her and DH.
. Not sure what that was about but possibly due to there being almost no separated parents among DSD's friends.
DH overruled her but although I was disappointed and sad, I decided not to go as it was about DSD, not us and I wanted to avoid awkwardness for DSD on her special day. I did explain to DSD that I was sorry to miss her special event (no mention of reason of course) and we shared the excitement in the buildup in the usual way and had a long chat after about how the party had gone, while eating lots of birthday cake
.
Would it be possible to have a few of her friends to a special tea some time before or after your DSD's party? Or perhaps a day out with a friend or two as a treat? Not to compete but just as an extra celebration?