Not necessarily from the start, but when he put down the pen tip to the paper to write the script he should have taken Irisu's words with a grain of salt as well, just like everything else he had heard (like the theories from the other film club members). The other Classics members still have no idea Oreki (possibly) failed due to falling to the Empress' machinations, yet they all realised the result wasn't what Hongou had in mind. So, nobody can say Oreki didn't have enough facts in his hand, had he been able to stay objective.

You are correct about this not being a crime scene and in the end the movie was absolutely more interesting this way than how Hongou would have liked it to be, but these are separate things from Oreki's image as a detective (or at least a puzzle solver) prodigy that he had just started to believe in himself. Besides, I feel there's some discrepancy between how disappointed the others really were and how bad Oreki simply felt himself, regardless of the others. Even if they appeared condescending or whatnot, they didn't even gang up on him or anything of that sort, and immediately afterwards they have been doing their best to either cheer him up or make him forget the whole thing (the pool ep).