It is more crowded later on the month... HHN should not affect crowds during the day.
I've been on opening weekend several times and park crowds prior the event are low.
OP - good question. The locals would know best on this one.
I've only done weeks surrounding Columbus Day, so my experience may be skewed by the holiday. imo it does get busier a couple of hours prior to close of business.
My assumption is many w/APs pop in before gates close to enjoy a ride or two pre HHN but curious to hear what is general consensus throughout October.
The Studios gets busier after noon with folks coming in for HHN and riding a few rides first. If you are going in the morning, you won't see a lot of difference but the afternoons are busy.
Depends on when you are going. Early in the season, there are only a few hundred people staying for HHN, which is insignificant. Regular park goers tend to clear out around 3, so I've always found the few hours before the park closes for HHN preparation to be as uncrowded as you can possibly get it.