Hard drugs, once legal or semi-legal as you imagine, will be marketed heavily. They will be as ubiquitous as alcohol.
Cocaine Slurpees, here we come!
As I said, you'll notice they don't sell alcohol Slurpees at 7-11, or any alcohol to any kids anywhere.
There will be lots of police around to enforce the incoherent web of legal and semi-legal laws that you envision.
Regulated is not "semi-legal" nor "incoherent" - and there are not "lots of police around" to enforce regulations on the legal drug alcohol.
Weak response = no response.
Alcohol kills 75,000 people a year in the United States, including 10,000 highway deaths. Lots of police and bureaucrats are trying to solve that problem.
So you want unconstitutional rules against Cocaine Slurpees? What kind of wishy-washy libertarian are you? Good libertarians are against all constraints on commerce. Let freedom ring!