I don't see what the big deal is. It might get their attention more than the curriculum. How much language do kids in a Chinese public school class actually produce anyway? At the very least, I'd argue that the time spent watching a movie is better than the same amount of time spent with a Chinese public school teacher using the audiolingual method or grammar translation. Finding a way to show kids that there's something interesting about a foreign language is probably good for their motivation and it sounds like you've taken measures to make it more comprehensible.

However, if you're showing the movie over the course of multiple classes, I'd suggest breaking it down into chunks more, so that there aren't any days where they aren't receiving explicit instruction. You might also want to think about using English subtitles if you're not already. I wouldn't hesitate to drag it out for a long time if the curriculum you're using is a series of grammar points exemplified by things that people wouldn't really say or write.

I think people will give you shit about showing movies for a lot of reasons. Chinese parents probably have a certain idea of what education entails and might not like something falling outside of that. Other teachers might give you guff because it doesn't take a lot of effort, as if the amount of effort you put in is somehow indicative of how much the students will learn. However, the main thing that I'd be concerned about is how the students react to it. While it sounds like somebody wrote the letter for this student, could it be that the letter stemmed from the student complaining about not understanding the movie?