[Feyn]

I think all the clues given here are not related to the movie.

Spoiler for episode 9:

i think all the clues in this episode are there to know why Hongou can't say who is the culprit now.My guess would be due to the ad libbs, Hongou can't solve the crimes anymore(in the script,we don't know if the dead has his hand cut for example).That's why they have to find another solution(and ask Houtarou and the others).

Otherwise , the possible culprit would be

Spoiler for possible culprit:

Sugimura, the boy with glasses.He's the only one who has a clear view on the entrance and see the others go to the rooms where Kaitou(the victim) is.He would go where Kaitou is ,kill him and the most important for the mystery,he takes kaitou's key and replaced it by his own key then locks the door. Oh and another thing, to counter the prop master's hypothesis, they claim that the window is too hard to open, but why does the window have to be closed in the first place?