Again known history: the one thing that the Civil War proved was the Supremacy of the Federal Government.
If a convention were convened, and the delegates decided on a Parlimentary State as I have previously conjectured, why would they need their new proposal to be ratified by any states?
Could they not simply decide that approval of the U.S. House of Representatives would be sufficient to enact this new device they created?
After all, the Federal Government is Supreme. This, I believe, is something the founders would have found objectionable. If they could have foreseen the events of 1861-1865, they might not have written the constitution at all.
The federal government is supreme? I do not believe that at all. I believe the people are the ultimate deciders.
As far as what prevents the convention from simply deciding that ratification of the House is all that is needed? Again the answer is the people. The people are supreme and simply would not allow it. They would lose consent of the governed and the whole thing falls apart.