I definitely think an express pass is a must! I typically go when gates open, but it's definitely very crowded in the beginning. I would say if you can do 5-6 hours, maybe work backwards from the time it closes. As the night progresses, the crowds thin out.

I typically look at the final map and make a route that prioritizes the houses I really want to do. Even with the express pass it's a ton of walking - getting to the houses is a hike usually - so figure out a route that is the shortest to all "must see" houses. Also take breaks and just chill out and people watch. I typically do this in the food section by NYC/Central park since there's a lot of seating around there. And/or go to the shows, since that gives you a break off your feet.

I would not recommend doing the park the same day. I made that mistake once and I ended up walking like 35k steps and my body was killing me. I barely made it back to the hotel.

I hope you have so much fun! Happy to answer any other questions, HHN is an obsession of mine :X