I just realized the OP's post was from last year ...
We (my family of three along with another family of four) were there about that same time. From our experience:
1) Do the parks hit capacity during this time?
Don't know if that's ever happened. I suppose it could - they were very crowded, as to be expected, but I never saw the gates closed at any point. The Studios side has all the actual NYE countdown and activities, so it stayed open much later than IOA that night. Pretty crammed with people, but not overwhelming, and well behaved IMHO. Also, I have been told by several resort employees that hotel guests are always assured of entry, so I assume that is actually true.
2) Are the Express lines significantly longer this time of year?
Longer, but certainly manageable, and worth their weight in gold.
3) Any tips for visiting would be appreciated.
Make dining reservations as soon as you reasonably can, and expect fairly long waits for service, especially at sit-down places. On NYE we had ressies for 7PM at Finnegan's but weren't seated until well after 8 (but were served quickly once we got our orders in). Consider eating at off hours (or even at the hotel, if possible).
With EP we easily got on all the rides we wanted to multiple times, although even with lining up twice well before the gates opened at IOA for EE we couldn't manage a single ride on Hagrid's, as it was down at open and never got below a posted three hour wait. Longest we ever waited was about 40 minutes for VC either first thing or at close (great night ride BTW), and that was without it having express yet.
We spent half a day at VB. Weather was very nice, but given the season, going with EE means everything is a walk on basically until after lunch. Just a great park.
The Grinch meet and greet was a blast. That can be tough to book as well, so get on that early.
To us, the parks are just so beautiful that time of year. UO goes all out for the Holidays and we loved being there.
