HHE is the 999 Happy Haunts Event, special ticketed event for Haunted Mansion fans and collectors. I'm not sure there will be huge amounts of people in town for it.
The MNSSHP on Halloween night is apparently usually sold out and very crowded. I haven't been, but haven't seen any long lines in the parks on Halloween day.
According to the itinerary, the MK party for the HHE starts at 8:30 on the 27th, so the 7:30 closing may be correct.