I think that your 4th reason is sufficient, by itself, and stronger than the others combined. If the Constitution forbids the government from waging such a prohibition, then the government is forbidden - regardless of social costs, relative toxicity, etc.
http://www.freerepublic.com/focus/rlc/738016/postsI disagree, simply because too many judges see the Constitution as a living breathing document.
It's far more logical to compare the substance itself in comparison to tolerated drugs to examine why the substance is illegal in the first place.