The point ctdonath was making was that the Supreme Court has ruled that private, personal cultivation of marijuana, even when in accordance with state laws, has an effect on interstate commerce. The relevance of the effect on interstate commerce is that the activity - growing marijuana - falls under the purview of federal law and federal law enforcement agencies.
To bring it back to your suggestions and the distinction between conventional and legal arguments, ask yourself this: for each suggested argument, think of two worlds with one being a world where your argument is absolutely correct and the other being a world where your argument is absolutely incorrect. Is there a difference between these two worlds in whether or how personal marijuana cultivation affects the illegal interstate marijuana market?