If I get a day pass during the weekday, am I allowed to stay all day then leave at night (not camping), or will I have to leave the park when the office closes at 4:30pm?

If I remember correctly you just need to close before the gates are locked, which is normally 10pm. However you may want to call the office to verify.

You have to leave before 10pm.

Come dusk you can't be by the overnight camping grounds but you can be the restaurants and dance hall