If there are people buying food and need to sit, it would be rude to do that. They DO sell food until the park is closed and seating in that area is already hard to find at any given time. I would not do that. Illuminations can be seen from anywhere in the park. They will block off the section only for people buying food before and during that time. The park does not always close at that time as they can have random extended hours or magic hours.
You may get away with it. Depends on sheer volume. I've been able to do it a couple times, but have also been asked to leave as well. I think the best place to watch Illuminations is from Japan. Lots of places to sit that look over the lake.
It depends. If it's a busy night at the parks and other guests are trying to find a table to sit at to eat, I've seen CM's rightfully clear out malingerers waiting for the fireworks. If its not that busy, you could probably get away with arriving an hour early, enjoy a nice long leisurely dinner and catch the fireworks with no issue.
Yes just be aware that people will try to join you at your table or even try to share your table
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