To answer your question, you can leave the MK and return to it later in the day using a one-day ticket without a hopper. To go to another park later would require the Hopper option. However, the MNSSHP is a separate ticket event.
The MK closes at 7:00 pm to guests who do not have a MNSSHP ticket. You would need regular admission to visit MK during the day, and then also the MNSSHP ticket to stay for the party.