I'm not sure why you said "even then". Chances are that if you see less than the whole story, things will seem lacking to you, not the other way around.
I'm not gonna agree, or disagree with your statement that (all?) characters were flat, generic characters. This is more than true for some, not so much for others. Draco being a decent enough example of the latter, he just never had much time to shine, partly due to reasons you mentioned.
But this is also why some people (myself included) feel hesitant about Draco being the MC in this. It just doesn't really fit his personality, depending on where it goes.
I said "even then" because I assume the movies probably cut out a bunch of stuff and only put emphasis on a few main characters, and showed their most important defining characteristics by highlighting key moments from the books. In the movies they were flat and they had literally no personality... I read the first book and half of the second book after watching the movies and I didn't see them get anymore depth than what was in the movies. Considering the fact that I read a book and a half and don't remember a single memorable or notable detail from them... I don't think there was anything in the other books that would have massively changed the perception I got of the characters from the movie. The fact that I read the first book and half way through the second one about 5 years ago and don't remember anything from them is quite telling to me due to the fact that I can remember most of the details and characters of almost every other book I read for the past 20 years. But anyways that is besides the point and moot at this point since my opinion on the matter has changed after playing the demo.
I wasn't aware the game was starting at the end of the series when I wrote my original post. I had not played the demo at the time, since I was just reading through the thread. I honestly thought that the game was going to start when they first arrive at Hogwarts... or a few months into their first school year. Which is why it made sense to me that Draco could / would be the main character... since you would have a chance to take him down an alternate route from what was established in the movies / books. After playing the demo I agree it seems weird to be playing Draco at this point and time since Hermione and Ron's relationship are already established, and Ginny and Harry are also... not to mention all the crap that Draco has already done, and how he was douchey to everyone. At this point in the timeline it makes it super hard for Draco to convince anyone to do anything willingly without straight up mind control. Even if he does what the ghost tells him to do... by driving a wedge in Hermione and Ginny's relationship with their boyfriends by giving them negative perceptions after dreams... the likelihood of them willingly overcoming their dislike for Draco even with magical assistance is unlikely.