Early Access is a perk for the most expensive AP, and I think you can also "buy" into it, but this was a little unclear to me. I can answer most of your other questions, though!
There is a special line at the front for "early access" ticket holders, whereas the hotel guests are dropped off at the Disneytown Entrance.
I actually waited with the hotel guests, (I had stayed at
Disneyland Hotel) initially, but had to go to the front of the park to use my AP. Once I got in, lines were still very low, along with crowds. It still seems very beneficial, to get in early. I *think* the hotel guests got in at the same time as the Early Access tickets, but I'm not certain. I was a little early, and waited in the Disneytown line first. At 8, they started letting people in, and I think park open was 9. I had to walk over to the Front Entrance and "join" the Early Access line instead, though. By the time I got there, there wasn't much of a line for security or to get in, but that might have been because there weren't many people in line and they're very efficient, rather than because they got in before the other people did.
I imagine they still limit the number of Early Access tickets to keep lines short and low crowds. Right now, there's no way the hotel guests alone could "fill" the park, so the EA is a way to maximize the cost and benefit while still providing a benefit to hotel guests. It was a Sunday, but I went on Soarin', 7DMT, Tron, Buzz, Peter Pan, MAWP, and Pirates in the first two hours, mostly due to that hour head start on the "big rides". Once you're in the park, you can also get fastpasses on the app, which also helps "maximize" the usefulness. Soarin' was a fastpass for 9:30, and then I was able to get another one for the River Rapids for 11:30, and then, another for Tron in the afternoon. Later in the Day, Soarin' and 7dmt hit 100+ minutes, though Tron stayed low(Which is a good gauge of how big the crowd gets).
It's still pretty new, though, so it will be interesting to see if it becomes more popular, and just what the limits are. With the AP, I have to "reserve" my early entry, so there's definitely some limit in place.