One week and one day from now is Jan 6th, right? The first two weeks of January are normally very slow. I would wait on express pass like Metro says. Normally HP is the busiest first thing in the morning. If you can stand to wait, head over your first day after 4pm and check out the crowds. If it is too busy then go first thing the next morning.
Remember for FJ that if the line is too long you can go through the single rider line then go back and walk the castle tour to see inside. You do miss a little but not too much.
For the "what time to arrive" question: personally, I like to be pulling onto property 1 hr before opening. That allows time for any lines to park, walking into the park (10min), stopping for photos, potty breaks, and still getting into the park before the "rope drops". You can certainly arrive later but I don't like to rush.