If you don't like to feel trapped in the sense of tons of people in one area, NYE might not be for you. I was at MK last year for NYE and we were on the bus by 8am. Our driver told us to look out the windows and the buses and cars we saw? They were already in 5-6 hours of traffic trying to get in MK. He took some back way and we made it there very quickly, but it was absolute gridlock for miles on the roads in from what we could see. It took over an hour to get from Liberty Sq to Tony's on Main St starting at about 730pm. Because we never left the park I am not sure what crowds looked like trying to get in, but it couldn't have been anything pleasant. I would guess if you were able to enter MK around 7, you probably would be stuck totally on Main St or fighting your way around the crowds for over an hour, like we were, to get anywhere else. We couldn't even successfully walk through the shops as a cut-through because they were so mobbed.
If you don't like to feel trapped in terms of staying in one spot in a park, that's different. Everytime we saw crowds we turned and went the other way so we ended up doing a lot, and the only times huge crowds truly affected us was at night near Main St. It was honestly so crowded I had a panic attack once we got out of the situation and to the restaurant. Yes, stupid me made an ADR for 8:30p at Tony's on Main St on NYE, but it was the only thing we could find for days so I had to do it.
Depending on what park you want to go to, I'm not even sure what the traffic situation might look like, let alone the front gate by later in the day. We were told it had closed twice that day but I have no idea at what levels the closures were. This is just my perspective from inside the park on NYE since as I said we never left....I just can't imagine that at 7pm or so it would've been anywhere near easy to enter MK. We took the railroad and there were just gobs of people at the gates.