Will I be able to see the castle and the fireworks show from the Studios?
What happens if I have the ticket to the Park for the day and I don’t leave it by 6:30pm, will I be forced to leave by someone is there a fine for that?
Well, from my experience on the 2019-2020 NYE party, as Karin1984 says the party starts at 8, but the park "closes" at 18:30; however, party ticket holders may enter the park starting at 17:00.
What this means is: After 18:30 if you don't have the party wristband, you won't be able to ride anything, shop or eat, as they ask to show the wristbands before getting on any line. That day, a lot of people stayed in Main Street USA and the central hub without the wristband until the NYE show... but taking into account how cold Paris is this time of year, I would NOT advise it. Also, they did have a showing of "Illuminations" at 18:00 that day.
If the Studios park is closing at 00:30 that day, then they'll probably have a NYE countdown and fireworks show using the projections over the HTH tower, so you'll have a NYE fireworks also. That year (2019-2020), they allowed every guest with a standard ticket to DLP and no party ticket, to jump over to the studios after 18:30 to welcome the new year on the Studios park. I'm guessing this was to encourage people to leave DLPark as they were in fact converting your ticket to a park hopper after that time.
As for fines, I highly doubt it. As Karin mentions, the worst thing that can happen is that security escorts you out. However, this is something I didn't see happen on 2019, but it becomes very very unpleasant to not be able to shop, eat, or ride anything. You can only literally sit around and wait... for around 6 hours... until the NYE show starts.