Did they invent some new legal system just for the Constitution? Where is it written in the Constitution that non-traditional methods were to be employed?
Article VII of the Constitution provided state ratifying conventions with the option to ratify the Constitution as written. And each of the states did ratify the Constitution as written.
The Ratification of the Conventions of nine States, shall be sufficient for the Establishment of this Constitution between the States so ratifying the Same.It does? Where?
Actually, yes, the Constitution did contain some novel concepts, but that's not my point. Aside from the whole notion of attempting to apply concepts from the law of contracts to the ratification of a Constitution (which, despite what you have said, has never been "common practice"), the particular concepts from the more modern law of contracts that you are attempting to utilize (i.e., piecing together a contract from a portion of a document here and a portion of a document document there) did not even exist in the eighteenth century. It wasn't even until the late nineteenth century that we began getting rid of the rules requiring particular forms of action wherein the facts had to be somehow tortured and twisted so as to fit the format of a particular writ.