As for the fastpass thing, the prior posters are absolutely correct in that you won't need them. For Philharmagic, there is no big line or crowd.
As for whether a 2.5-yr-old will enjoy it, if the child knows/recognizes some of the stories/characters involved (Donald, especially, but also Lumiere, Ariel, Jasmine and Aladdin, Simba, Peter Pan and Tinkerbell) they should enjoy it. The theater does get dark, but there is nothing really scary in the show once the champagne corks are finished popping and the big broomstick is finished stomping across the stage. These were the only two iffy moments for the just-under-three-yr-old who was with us last trip. In the end he loved it, especially the finale. He's a huge duck fan in general, whether Donald or otherwise. He liked helping Donald keep up with the hat.
If it gets intimidating, but your child is not upset/needing to leave, try just watching without the glasses on. It's kind of out of focus, but there is no 3D effects without the glasses so it is less intense. We did this during Muppets when something (still don't know what, specifically) kind of freaked him out.
If you're still in need of help/information in general, let me know if I can help.
Have a great trip!