Were signatory to the Single Convention on Narcotic Drugs, which if I am not mistaken you once mentioned as being the reason why Obama could not legalize MJ, although Administrations have the power to unilaterally change schedules at the bare minimum without Congressional approval as explicitly stated in the convention and the CSA. Perhaps I am mistaken about you making that point.
But legalization? No. States do not have the right to supercede federal law promulgated because of international agreements.
As far as states not having to cooperate I don't know how absolute that is. At some point aiding and abetting might come into play but I am not certain about that. I do wonder why one should consider law binding at all except from a punishment perspective. If a state can obstruct justice then I'm not sure why an individual has any moral obligation either. But law is law and has no morality or "right or wrong". It's the rules, no more no less.
Without reading the treaty my strong bet is that each country is required to ban marijuana in some way, not that each subdivision must enact its own separate ban across all levels of government. (That would seem to be an almost impossible provision to enforce)
I think the crucial difference there is between not banning something and obstructing someone else who has banned it. States would not be able to thwart federal law enforcement that wanted to arrest someone for possessing/selling marijuana but they do not have to participate in that effort.
More importantly though, if the treaty DID say that such a provision would be unconstitutional. The federal government does not have the power to force states to pass laws and it does not have the power to force them to enforce federal laws. That's why the Feds often tie compliance to funding as a carrot to get states to play along.