MK doesn't close until 6, and anyone who is attending the party is allowed to stay in Tomorrowland until the party officially starts at 7, so no, you wouldn't have to leave and come back. However do be aware that if you are entering the park at 2, you will need to have admission other than your
MNSSHP ticket in order to get in. Your party ticket only gains you admittance to the party, although usually they start letting people in around 4 pm.