You could perhaps prove that the Gambit went down because of tampering, but actually tieing it to someone would be virtually impossible. I am quite sure that Merlyn didnt do the deed himself.
Do we know how many people have a book? I think we have only seen two of them both coming from the Queen household. Holding Oliver hostage doesnt mean he would ever be released but merely used as leverage when the time is right. You dont burn resources if you dont need to.