I've had a coworker in the past who was honest about his drug use, and it took him four pages to list all the times he'd used drugs. He did not get a clearance.

First things first: I don't think you're going to get a security clearance. If it'd just been pot that's one thing, but even then they prefer it not to be so recent. But if it's more than pot, I doubt that is going to fly, even if you were a juvenile. You were recently a juvenile, after all.

Next up: I actually doubt they're going to take your security clearance as a chance to file charges against you. I'm not positive, and if you said you sold drugs that probably raises the odds, but I would give you odds that they won't, actually. Just a hunch.

Third: you shouldn't lie on a security clearance form, you're right. In your case, I probably would have simply not gone for a job that required a security clearance, though. Incidentally, did you note on the form if it had some sort of caveat like 'within the last 7 years' or anything? Because if so, you could reapply in the future and not have to divulge. Though, I think it's just pot that they don't care about if it was more than 3 or 7 or whatever years ago, I think they still want you to divulge all other drug use.

Fourth: They're not going to ask your friends 'what was so-and-so like when he was on meth?' They'll ask a number of questions and one will be 'did you ever know so-and-so to have a problem with drugs?'

Fifth: I have no idea if what you put on the form, or say in a polygraph, could be used as evidence against others. I actually doubt the investigators care to pursue anything, but I would've not specified names.

Last: Pretty sure that just buying something from overseas doesn't count as supporting a foreign natural, or else everyone would be guilty of supporting China. They want to know if you have stock in foreign countries (not mutual bounds that might contain foreign companies, but direct support of a single country), or send money to relatives over there to help support them, or have funds in a foreign bank. Stuff like that.

Source: Have held a security clearance for 20+ years (though never been involved in drugs so that hasn't come up on mine).