And Hermione's parents enter Diagon Alley, would they definitely need the presence of Hermione to do so?
I think the answer to that is that they very definitely would need Hermione there to be able to enter. She has to tap the bricks to get in for one thing. With Hogwarts and something like the QWC, I'm not sure they could enter. It's possible that all they need is Hermione telling them that the Muggle repelling spells are false and Hogwarts isn't an old ruin, but it's also possible that they just can't enter.

I'm not sure they can get on platform 9 3/4, either. Lily's parents are definitely at King's Cross, but it's not clear which side of the barrier they're on when she talks to Snape.