You know, I never thought about it that way before, but Carlotta may very well have come up on the spot with the excuse that the room was Mrs. Stoddard's when she was a little girl. And the reason that may be likely is that it's made clear in the script that the room was Sarah Castle's and, of course, Carlotta would have known that (and the audience would have learned it, too, if in DC's infinite wisdom and apparent dislike for Monica Rich's work, he hadn't cut the scene that would have clearly explained that, but no
). Plus, Carlotta didn't want to tell Quentin that truth just yet because, as Grayson Hall notes in her script, Carlotta doesn't think Quentin is quite ready to hear it.
As for Quentin, he may accept the Mrs, Stoddard's room explanation and the trick of the light theory because it seems as if he might feel a bit embarrassed and because of that he wants to make a graceful exit from the room without appearing more foolish. It would be interesting if David Selby made any notes to that effect in his script, but I have no idea if he did or not because I've never seen his script...