Yes, you can enter the park before 7pm - infact, technically, they don't actually shut it...well, they always done it that way previously - people who are there from regular park hours can stay in the park and still shop etc. Just the rides shut down. Ideally what you should/could do is book a PS for dinner at the MK for something like 5.30 and then you can stay within the park out of the way whilst it's decorated. This year they are giving people with
MNSSHP tickets the option to book PS at the Liberty Tree Tavern NOW (instead of at the usual 60 day mark)
2 years ago, I booked Crystal Palace for 5.30. (We entered the park before official closing and exchanged our tickets for the wrist band with the CM's that are especially stationed by the little garden in front of the Castle where the Walt and Mickey statue is) We finished our meal at about 6.30 and by the time we left, the decorations were all up and people were already entering the park down Main St in there hoards - so it was well worth it!
You won't have any problem with parking.
Penny