Don't cancel your reservation as if used correctly many praise this event, but I will give you a little advice. The check in time listed is late to what time you may actually check-in. Often times they're set up at 9:00 or earlier and ready for check-in. At that point you're able to pick your show time with hardly any issues because there should be plenty of each time slot open. Showtimes typically don't start running out till the 10:00 mark.
There is a large area roped off for both the parade and fireworks just for package guests. The area will be the same for both morning and evening events. And again when checking-in in the morning for your reservation I would ask the host what the recommended arrival time to your reserved area is and follow their suggestions. Opening for parade viewing area may occur 15-30 minutes early prior to 10:30, depending on the day. Package guests will have a reserved section for the 11:00 parade. The typical opening time for fireworks is 8:00 not 8:15. Often times the people on the phone know little about how the event runs, your check-in host(s) will give you all the inside scoop. And checking-in doesn't mean you must stick around till the event starts, it simply entails picking up your credentials and selecting your showtime. And finally yes it is very possible to do both Fantasmic and the fireworks you'll just have to see the second showing of Fantasmic.
