My 0.02, you should really pick a different profession or at least be smart enough to not get caught if you're going to be doing drugs because your going to have a lot of explaining to when trying to get a job in future. From what I'm reading there, I don't see any reason you shouldn't be able to take a written since you aren't applying for a rating until you go to take a practical exam. You should also inform the FAA this has happened, however I would at least talk to your AME or better yet an attorney before you do that. The regulation your quoted also said "is grounds for", which to me sounds like they could use this as the reason for denial/revocation if they want to. The one thing you have going for you here is it doesn't sound like you have a license yet so at least you won't have a revocation on your record:)