Hi there! I'm not sure that my experience was exactly the same, or will be something you want to hear, but here goes:
I had to wait in line for quite a while before going into the stadium. Now, granted, I'm not one, generally speaking, to stand in line or sit on a curb for a couple hours waiting for a show or parade to start. So my friend Betsy and I probably arrived at the stadium about 30-45 minutes before showtime. There was a huge mass of people waiting, and the line wasn't moving at all. Eventually, we were allowed into the stadium, wound up getting pretty good seats (about halfway back from the front row) and enjoyed a fabulous show. So I'm not really sure when you can go into the theater. Waiting in line wasn't a whole lot of fun, especially as there were a whole lot of impatient kids who, as one might guess, got a little whiny, what with there being a whole, big, wonderful world of Disney out there, and them not understanding why they had to stand in this rather crushing line.
Good news: Once inside and seated, we still had about 15 minutes to go until showtime and the crowd was having a lot of fun, doing the wave (this has happened three different times when I went) which was silly and fun. And once you've secured seats, you can always send an emmisary to the snack stand for food, or you can wait for the vendors who come around selling popcorn, etc. I don't think it would be wise to try and leave the stadium for food, shopping or a ride, as they might not allow you back into the stadium once you've exited and it's gotten full. On a more personal note, I'll admit that I'm not a big fan of people saving seats for someone who's off enjoying the parks or whatever. It just strikes me as unfair to all the people who are sitting, trying to keep the kids entertained or just patiently waiting for the show. Not to sound all Scrooge-esque or anything!
Generally, I like to grab a turkey leg on Sunset Boulevard before diving into line. Fun to eat, if a little messy, and by the time I'm done with it.. hey, the line is moving!
Word of warning: Be prepard, while you're waiting for the show to start there are a huge number of vendors trying to sell you and the kids everything from glow sticks to souvenier cups. It's pretty relentless!
Richard