Andrew, in (rightly) rejecting the analogy between the condition of the EU and that of the United States before the Constitution, you write:
For all the differences between them¸ the dominant elements in the newly-independent American colonies were (broadly speaking) united by more than they were divided, not least a shared language, heritage, the common law tradition and so on. They had also recently won a shared victory against a common adversary. The European project, by contrast, was specifically designed to reconcile the (previously) irreconcilable.
No less an authority than Publius himself is on your side. In Federalist #2, John Jay pressed precisely that point against the opponents of a stronger American union:
Providence has been pleased to give this one connected country to one united people — a people descended from the same ancestors, speaking the same language, professing the same religion, attached to the same principles of government, very similar in their manners and customs, and who, by their joint counsels, arms, and efforts, fighting side by side throughout a long and bloody war, have nobly established general liberty and independence. This country and this people seem to have been made for each other, and it appears as if it was the design of Providence, that an inheritance so proper and convenient for a band of brethren, united to each other by the strongest ties, should never be split into a number of unsocial, jealous, and alien sovereignties.
The history of Europe — ancient, modern, and even contemporary — doesn’t exactly offer an echo of this.