Translation:What is this movie about?
I believe so, however, 个 is kind of a "catch all" measure word, is usually used with 这/那 to mean "this"/"that"
It is more accurate that way, but both are accepted for the aforementioned reasons.
Yes, but since "movies" emphasized here are considered "individual objects", they can come with more than a classifier. So either 部 or 个 can work.