Your own quote shows I am correct. “The Ratifications of the CONVENTIONS of nine States,...” These were special institutions set up in the States for this one purpose. Do you think I am denying that States existed as political institutions?
Congress called for the Convention and established the rules of ratification. It also was tasked with receiving the finished document. What else could you have in mind?
I obviously did not quote a section referencing conventions to argue against conventions, though I am perhaps being generous in my assumptions of what is obvious. That you would resort to such misdirection is telling of your character. It is not the mechanism of the convention which is in dispute, it is the issue of state sovereignty.
There is nothing about the convention process which witnesses against the sovereignty of the state. That the ratification proceeded along state lines evidences their political sovereignty. If, as you seem to suppose, they were merely seeking the approval of a larger, whole national population through expediently localized processes, why then would they have bothered with state identities? Why, instead, did they not pursue this piecemeal ratification along the more natural lines of common geography and solicit "Blue Ridge Folk" or "Chesapeakers" or "Adirondackers"? Why did they not treat along ethnic lines and seek the approval of Scot-Irish or the Old Dutch? Why not break down the population according to religion and have separate conventions for the Quakers and Episcopalians? If, as you claim, ratification spoke of the will of an aggregate, then why seek the affirmation of nine states and not a like majority of the general population instead? Yet they insisted in recognizing, as unitary political powers, those pesky states (please investigate the word "state") which confound you with their persistent existence before, during, and after the Articles. I wonder, do you hold that France was created by the League of Nations, all evidence to its preexistence and continuance not withstanding?
A consistent application of your theory of demonstrable sovereignty would hold that the Irish, having engaged in a popular referendum to determine their nation's adoption of the European Union constitution, actually DISPOVED the political sovereignty of Ireland. You seem to believe, when it accommodates your pre-conclusions, that sovereignty is expressed through a singular government such that the extra-governmental ratification process which took place in each state disproved the very political self-determination upon which the very matter of the choice depended. The contradiction necessarily arises that one proposition, that the states are shown to manifestly NOT be sovereign by the action of the people, is used to support a second proposition, that a larger nationhood IS sovereign through the same popular action, even (and especially) when it is tediously state-defined. That is, you claim that the sovereignty of the states was disproved by the direct action of the conventions as opposed to the agency of the state governments *in order* to prove the sovereignty of a greater nation, the circumvention of its government not withstanding. For you, the identification of "the people" is hinged to government in such a way as to allow it to swing whichever way you think you need. You either believe in "popular sovereignty" or you don't.
Not only did you misplace the emphasis to highlight "CONVENTIONS" and not the pertinent "OF NINE STATES" (in an inflected language, this would demonstrate the genitive of possession), you left off the part which is absolutely fatal to your argument. I'll reinstate it: "..., shall be sufficient for the Establishment of this Constitution between the States so ratifying the Same." That is, upon ratification, those states pending membership came under the constitutional supremacy of the United States Constitution. As a corollary, any state NOT "so ratifying the Same" did not. So if, say, Pennsylvania had been a lone dissenter and never ratified the Constitution, it would be outside the union. If you do not agree that an independent Pennsylvania would be a sovereign Pennsylvania, and that states were therefore initially invested with sovereignty in order to be able to parley with it, then you are beyond reasoning.