I didn't get much traction on this thread, so I'll answer my own questions for future DISers with similar concerns:
1. Since no one is saying "gates WILL be closed by xx time", I'm going to interpret that as "gates may close occasionally, but not all night". Is this correct? If the gate closes and 10 people leave, do they let 10 people in?
Gates did not close, as far as I could see. I did hear that Disney had at least one park closure. I think a few things contributed:
a. USO is a popular destination, but still not as big as Disney. Give it 5-10 years, and I think you'll see similar crowd issues though.
b. USO has fewer onsite hotels feeding their parks (for now).
c. USO worked HARD at crowd control. They had multiple paid parties going on all over the property (Citiwalk and multiple hotels). This seemed to draw certain demographics to preplanned areas.
2. Roughly how long ARE the lines for security (at RPR) and at the gate? 1 hour each? More? If we get a 5-6 hour break, 2 hours for these lines is fine. But if we only get a 3 hour break, and the lines to get back in will also be 3 hours, that's a different scenario.
We were onsite, so security was either at the RPR water taxi or on the Garden path between RPR & Citiwalk. Either way, both were fast and painless, as usual. Even the morning (about 45 minutes before EE) was light. This was true from 12/31 (Tuesday) through 1/2 (Thursday), with 1/2 being the busiest.
The lines at the IoA gate were also reasonable - less than 10 minutes. US may have been busier.
3. Would it be better to head into IoA until 10 and then take the train over to US? Or just go straight to US at 6 pm?
We went back to IoA from 6-7:30, then took the train over to US. We had our express passes to put us on the train with minimal wait time. It honestly felt like any other (busy) day.
4. We do have a late dinner reservation in Citiwalk. But I'm planning to put in our name at one of the US restaurants when we get there, hoping for a 2-3 hour wait (this would be if we go straight to US). Is that crazy? Should we just jam some granola bars in our pockets, add butterbeer, and call that 2nd dinner?
I was able to snag a last minute 8:15 reservation for Finnegans, so I dropped the 11 pm Cowfish. Online cancellation was very easy. When we got to Finnegan's, standby wait time for parties of 3-4 were about 1.25 hours. Seemed perfectly reasonable to me, and that would have worked for our family just fine, if I hadn't been able to get a reservation.
5. If we end up using our Cowfish reservation, what kind of midnight experience is there at Citiwalk? We don't have tickets to EVE and we wouldn't be likely to go back to US after dinner. Is Citiwalk walled off in some way? Would we be able to see any kind of fireworks at midnight?
I'm embarrassed to say, we didn't make it to midnight. It was just too long a day, and only my husband was able to nap in the afternoon. So, we left around 11, and even though we didn't eat at Cowfish, I was actually able to see the crowd situation.
The waterfront area was open to pedestrians, but there were clearly some kind of barriers for most of Citiwalk. The crowds were massive, and lights were everywhere, so I couldn't get a clear view of how they were managing crowd control. What I CAN confirm is that, at 11 pm, we had no issues leaving US - the crowds coming in were heavy, but not a solid wall. We walked at a normal pace to the exit, over the bridge, and down to the water taxis. There was a line to get into US, but nothing like park opening. There were PLENTY of others leaving the parks, and very few looked like they were headed to the Citiwalk event (we were all plodding along with that characteristic end-of-a-full-park-day vibe).
There was lots of energy, great weather (nice and cool, not raining), and it really looked like a spectacular time for those with more energy than us. Not recommended for folks with claustrophobia or general dislike of heavy crowds. But if you know what you're in for, looked like a great time.
6. Based on all the above, and given that this particular plan does include a midday break, do you recommend we head back even earlier?
We toured from 8 to 1, with lunch at 11 (Leaky Cauldron, 5 minute wait). Took a break, unpacked, read a bit, rested up (theoretically), and headed back at 5:30. In retrospect, I wish we had waited until 6:30-7 - we may have made it to midnight then. Neither park seemed unreasonably busy, especialy by 1/2, when both parks were just jammed with people. Now, both parks were definitely BUSY, but you could walk through the crowds, you could use bathrooms, you could get into the shops.
Other notes:
Overall, NYE was not as busy as we expected. It WAS busy, but I think there was a good flow in and out all day as people wisely paced themselves.
New Year's Day was quieter to begin with (EE was amazing, if you weren't in the Hagrid line - and we weren't), and pretty busy by evening.
1/2 was the busiest yet, with dense crowds in Hogsmeade by 8:30 (EE at 8 that day). It was difficult to get in and out of HP shops in Hogsmeade by 10, and the rest of the park seemed packed that day.
On NYE, they also had 2 celebration zones set up at US to distribute crowds. They had many, many CMs in WWoHP to force crowds to walk on the right, prevent them from using the "shortcuts" behind the buildings, holding pens for the nightime castle show, and well-planned extended queues for Hagrid's Motorbike Adventure. The bridge from Lost Continent to Jurassic was used as a one-way funnel for the HP crowd leaving the castle show. The crowd control efforts were very effective.
I think the biggest shock was watching the wait times on Friday 1/3 drop significantly. We saw 90 minutes for Hagrid's for most of the day, whereas the day before was solidly 210-240. FJ went from 140-150 down to 60, and Gringotts was similar. We were home by then, so I can't speak to how crowded the parks really felt, compared to 1/2.
I actually wonder now if the "high crowds" were tucked into lines most of 12/31-1/2, and that's why the walkways were manageable. As wait times started to drop on 1/2, the crowds were more in the walkways again. Most crowd calendars are looking at wait times, not people per area. With EP, wait times don't really have the same impact.
So, we survived. The advice to go on quieter days is valid. If you must go on a high crowd day, plan to have EP. Use EE if you can, and definitely bring a healthy dose of patience and good humor. But no need to panic about getting into the parks or doing things.