For a 9AM opening, they let everyone in the front gates in the 7:45ish-8AM range. Exact opening procedures can vary day-to-day, hence the range. I’d mentally circle 8AM just for a nice round number.
You are free to walk around Main Street up through the Hub for that hour. The Main Street Bakery is open as well as some other shops.
Yes, people start congregating at the “spokes” in the Hub. You can’t actually see the castle stage very well from many of the spokes, so those towards the front of each won’t really see the opening show.
The most popular / most crowded spoke will be the bridge to Fantasyland (towards Tea Cups) as that is where a large majority of the 7DMT rope drop crowd will be gathering.
The opening show (Let the Magic Begin) will be at 8:55AM on the castle stage. It lasts a few minutes. Once the show is over, they drop the ropes at each spoke and the park is open.
Note that this “Main Street/Hub open an hour early” arrangement does NOT apply to pre-9AM regular park openings.
Hope that helps!