I love these threads, I find it so fascinating that people lead such different lives. Love the blanket statements about mealtimes, the shocked comments about primary children being out so "late" on a Sunday night!
Think I definitely inhabit a different world from some of the other posters!
Personally, I'd have no issue with the timing. Don't think it's that late for a 9yo tbh. I'd also have no issue with the lack of food, but I would want it to be clear on the invitation as we usually eat around that time and I'd expect a meal unless informed otherwise. It wouldn't be a problem for dd to eat a bit earlier as long as I knew in advance.
Don't think dd would be disappointed with the lack of food, if she had eaten already. She would appreciate a piece of cake in the party bag though. TBH, I'm really surprised at how many people think kids will "need" snacks if they're doing physical activity for 1.5 hours. DD does 1.5 hours of dance a couple of times per week and never takes a snack, nor is she hungry when she gets home. If people feel that kids can't manage without a snack, no wonder we have such a problem with childhood obesity.
It sounds like you have enough adults on hand too, with rink marshals and your additional family members.
The only concern I would have, in your shoes OP, is that I really wouldn't want to be left with a mountain of 50 presents. If I were you, I think I'd want to find a way of encouraging people not to bring gifts - maybe get your ds to choose a charity and suggest that people may wish to donate any money that they would have spent on a present on his behalf? Some people will probably bring a gift anyway, others may donate and others may not bother, but at least you wouldn't have to take a sack of presents home with you. DD only had 11 guests at her last party, and even then, some of the gifts ended up at the charity shop - I felt guilty, but what's the point of keeping stuff that they don't actually want?!