The frozen stage show starts at 9 on that night you'll be there. Fireworks will be about 9:15. If I were you with seeing Fantasmic 30 min before. I would sit toward the back of Fantasmic, or leave a couple minutes before it ends to beat the crowd out. Take the path to the far left that goes backstage and exits at the front of the park. You can then watch the stage show from the big screen above the stage, and then have a good view of the fireworks, and then also have an easy exit as you're right there.
You can try to get close to the stage but it will be packed by the time you get there. And for the fireworks, it's better to be a bit further back from the stage.