Didn't you guys hear the little line that snape dropped in Umbridge's office? (highlight):
He pretty much cleared Cho of being the "bad guy" for outing DA. Umbridge summoned him to her office to give her more truth telling serum to use on Harry, Snape said something about her using it on students- including Cho. Everyone in the room looked at Cho like, ohhh ok so you didn't tell on purpose...
Also, there are mutliple interviews saying that sub-plots were left out due to the confusion it would create. In novels, it's rather easy to carry on a sub-plot without confusing the reader. The writer can rely on chapters to separate stories and word usage to ensure that it's absolutely clear. That's pretty hard to convey properly in a movie....
I haven't read the books yet because I hate having to wait for sequential releases, so I plan on starting to read them once the final one comes out this week. However, I have read the detailed outlines on wikipedia haha
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