Yes, it's necessary. For you to enter the Magic Kingdom you need the ticket. You can enjoy all day in the park with the ticket and stay to watch the fireworks at the end.
To watch them in the Magic kingdom yes you have to buy a full ticket
Yes
Yes or watch it from a hotel
Best view for the fireworks, is inside magic kingdom park. You can see little from other places around the area, but it's not as good.
Yes if you want to see the fireworks within the park with full effects otherwise, you don't have to buy the tickets if staying in one of the resorts or outside the park.
You can watch from the Beach at the Polynesian
Yes- you may be able to see them a bit from outside the park, but unless you're staying in one of the hotels that overlooks the park, you're only going to get the full effect by paying to go in.
I believe you can technically see some of the fireworks from disney springs but it will not be the same spectacular show you see in Disney, so if you want to experience the whole show with fireworks then yes, you will have to purchase a ticket since it's location is in the heart of disney in Cinderella's castle.
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