If you are staying in a cabin, once you check in can you leave and come back late? Or can you not enter the park after a certain time?

The park closes at sunset, but if you are staying in one of the cottages night security will let you in through the front gate. It is better to get all settled into the park before sunset though

I stayed at the campsite. Seemed to be ok to come in as late as 11pm.

Park closes 6pm

Yes you can