No. SCOTUS cannot legislate.

They could rule that all drug laws are illegal, but they couldn't change the schedule for marijuana.

As far as I know, no US court would seriously consider a lawsuit that would invalidate drug regulation, so SCOTUS will never see such a case.

Yes, I'm not actually debating that. But the issue that I think the Court will need to freshly weigh in on is the fact that state law and federal law are in direct oposition to each other in many states now.

Now, of course the Supreme Court will come down and say federal government law supersedes state law, as the constitution specifies.

I'm no lawyer, but I do think there would be a nuanced way to argue that the states should have the freedom to self-regulate, as long as they don't distribute across borders. I know there is case law that has already said that the Feds have jurisdiction even in this case, but it still could be argued.