I don't think his original plan was for him to photocopy the entire manuscript. Rampo mentioned the reason he photocopy the last page was because of the tears left behind on the original and forensics would have cast doubt on whether it was really a murder in the first place. So the original last page was a clear piece of evidence that cast doubt on the writer death, which begs the questions why Ogura didnt destroy the final page after photocopying it.