1. You shouldn't have any problems booking BBB. Even if you do, visit Guest Services upon embarking. They hold aside a number of reservations. Our Magic cruise sails in 24 days and I just checked and there are still bookings available. They may not be at times you're interested in, however. (Again, just book something you think you can live with and change it onboard.)
2. I'm not familiar with the character dining options on a 7-night.
3. Here's a video so you can see for yourself what the Avengers Academy is all about:
4. I'm a vegetarian and I can tell you that you will eat well. There are lots of great options. I was extremely happy to have multiple choices every night.
5. As Shmoo said, check your cruise meet.
6. I recommend reading the DIS for lots of great tips. I also really like the
Disney Cruise Line Blog (which is not an official blog, but has lots of great information). I also found that watching deck tours on
YouTube and researching deck plans prior to my voyage last year on the Dream was very helpful to get a feel for the ship before embarking. Less time finding my way around = more time for fun.