We will have to agree to disagree JP, but we are not in a big disagreement.
I agree that under the Supremacy clause Congress can specify a lot of things, including requiring that a new law directly on the subject be passed regardless of prior laws, even by a specific method like "popular vote." What I don't think Congress can do is tell a
state legislature what method it must use to decide its position. In other words,
if Congress allows the state legislature to make the decision, if cannot stop the state legislature from delegating its decision to an executive authority (like a gaming board or a specific official).
This actually is not likely to ever be a real issue, but we lawyers like to keep our minds active with this stuff.
Skallagrim