For fireworks, the main question for you is how important are the projections to you. Castle hub will start having people a couple hours in advance and be mostly full nearly an hour before the show. From main street or the train station, you can usually still see the castle and depending on how many people there are, it may be possible to slip in 30 minutes before. They also play the music and do projections on It's a Small World but the fireworks get shot off to the left of the attraction so they aren't really lined up.
If you know the parade route, it's usually not too difficult to get a spot. Main street can be a bit difficult but if your group is small, the area between Matterhorn and It's a Small World is usually easier. Except on the busiest days, it should be possible to get a spot for the parade, even after the parade has already started. I sometimes watch the parade twice by getting a spot near the beginning of the parade route and then making a mad dash to the end of the parade route.