You have a lot of misinformation in your post. Statically speaking, states that have legalized marijuana has seen a decrease in teen usage. That argument is entirely nil.
Obviously there are health concerns regarding more illicit drugs, but those can be reduced with proper and safe dosages. Also, if the counter-agent is sold OTC, there'd be way less chances of overdoses.
I believe people should have the right to choose what they put into their own bodies.
Maybe, all my data come from Europe, and especially from France, where it is illegal. I also came many times to Holland (as it is the nearest place where it's authorized).
I was a bit aggressive in my message, because it's a concern that I'm really linked to. I hope I didn't offended you.
I agree with all of your ideas, about black market, danger about uses (asepsis, counter agent) and education.
But there are drugs that aren't nothing to do with human being in a common sense. I mean, do you know a drug called "krokodil"? I take an extreme instance to make me understand easily. The thing is that people that are coming to this drug are already addict people, and as they are starve about the product they used to take, they take this one indeed. I let you google for effects that this one had.
Now, I think there is really two kinds of drugs, recreative ones, that should be allowed, and dangerous ones, that aren't good at all.
This is exactly like drugs (I mean medicine drugs, as beta blockers, immunosuppressors, NSAIDs, corticosteroids, ...) which all have benefits/risks balance. Doctors use these with the knowledge that they could be harmful, and because of that, they are always used to try to get the maximum benefits effects, with minus harmful effects in a special case. But the thing is, that, in this situation, drugs are necessary to cure a disease or a state. So, they take risks because they know about benefits, and they always do it in the best positive balance.
I think it should be the same with drugs, but when it's recreational, it is only to fulfill a desire from the user, so there is no "indication" imo. No real situation that need to use these drugs. And there is still risks.
But with dangerous ones (heroin for me is a really a nice instance), you don't have "enough" good effects to counter risks, and even mid/long terms effects (as inward-looking attitude, paranoia, psychotic effects, skins effects, cardiovascular effects, there is so many...)
So yes, it is important to improve all the conditions to do it safely, but I'm afraid that it could "create" dependance that should be avoid in an other way. I mean, I don't really care that people try to smoke a joint. But that's a really different problem if they try a fix of heroin imo.
But I don't want to make myself judge of the devil "it could create new dependance blablabla", I know the pros and the cons, I'm just thinking as a good father, and I want the best for my children. That's all.