So since EA admitted students are also required to submit the FUN form, does this mean that if their grades dropped significantly or something like that, they can become rejected later?
Yes, EA admits can get rejected, although I don’t know if any ever has.
Yes.
When one accepts EA admission, it’s conditional on retaining good standing through the end of your HS senior year.
Although you may have been admitted, MIT will have turned away numerous other equally qualified applicants. Why give the school any reason to believe they made a wrong choice.
Yes. This is the case for MIT and other colleges. In addition to bad grades, other actions can lead to an admission being rescinded (which is the verb in this type of action, not rejected). One only needs to point the Wayback Machine to last year further down Memorial Dive to see an example.
http://www.thecrimson.com/article/2017/6/5/2021-offers-rescinded-memes/