However, the drug trade in general is archetypal interstate commerce. Almost nobody grows and processes their own weed or their own cocaine. It all happens through cross-border commercial transactions.
Going further, the purpose of the interstate commerce clause was to give the power to regulate tariffs (and other taxes) between the states, not make entire trades illegal because sometimes those goods are sold elsewhere.
For states where the state's laws agree that the drugs are illegal (i.e. most states now, and all states until relatively recently), there could be a (legal) federal war on drugs so long as the state government does not object. As to the practicality of the enforcement of such objections, we're seeing it play out right now with the states that have legalized it.
Er... really? In my experience, it's quite common.
where I'm from, growing and processing (curing and trimming) your own weed is very common.
this time of year is harvest season in fact.
plenty of odd jobs trimming marijuana pop up seasonally around here because of it.