Nothing you posted above discredits Madison's legal theory. However, you can start by addressing this question: If the words "general welfare" are a grant of general power (And not an understanding that the enumerated powers must be carried out in a manner that promotes the general welfare), then why did the Founding Framers bother enumerating any powers at all? No one in the history of the United States as been able to answer this question.
Among the enumerated powers voted down during the constitutional convention was the power to charter a bank, the powers to direct funds toward internal improvements, roads, canals, schools, and etcetera. And yet, the federal government of today in involved in all of this and more.
I'm a bit puzzled by your remarks, so rather than answering tit-for-tat, let me start with the question you posed above. My understanding of the mainstream view of enumerated powers and their relationship to the Tenth Amendment and the "general welfare" and "enabling" clauses is that the general welfare clause was intended to be a savings clause anticipating that changing circumstances would make some enumerated powers (like granting letters of marque and reprisal) moot issues an that new powers would be needed. While you note that a proposal to give Congress an enumerated power to fund roads was considered and declined, Article I explicitly gives Congress the power to fund "post roads". I often wondered why Ike went to a "National Defense Highway Act" to establish the Interstate system when we could make a reasonable argument for the need of expanded post roads. The Tenth Amendment was intended as a counterweight to this line of reasoning, and courts developed "balancing tests" to determine when changing circumstances justified a modification in what qualified under the "general welfare" and "enacting" clauses.
Now I realize that this is tantamount to a rejection of strict construction based on original intent. But without it, the enumerated power of providing post offices and post roads could not be extended to measures to facilitate air mail. Nowhere in the enumerated powers is there any provision for the federal government to purchase or lease airplanes to fly mail, nor for that matter to use trucks or railroads to transport mail, but everyone seems to hold that these are logical extensions of enumerated powers. Similarly
Article I gives Congress the authority to raise, fund, and regulate an army and a navy, but not an air force, nor a Coast Guard (although that agency grew out of Hamilton's revenue cutters of the Treasury when there was no US Navy!), nor any intelligence services; all of which are now deemed necessary parts of national defense and not to have required a constitutional amendment to add them as enumerated powers.
I think a fair reading of Madison would confirm that he anticipated that if the Constitution survived more than a generation (quite a few founders anticipated the project would have to be re-worked every fifty years or so), it would need to be tweaked, and he realized that the amendment process was too cumbersome to be often used. By his presidency the first twelve amendments had been adopted; the Bill of Rights in the first rush, the Eleventh to reverse
Chisholm v. Georgia, 2 U.S. 419 (1793), and the Twelfth to fix the electoral college problem that almost elected Aaron Burr president. It took a full sixty years and a ruinous civil war before there would be another.
In short, I believe that the "general welfare" clause was intended primarily as a preamble to the enumerated powers, and secondarily as a saving clause to lend flexibility to extend the enumerated powers without amendment when there was substantial consensus that evolving circumstances made such flexibility logical. The Tenth Amendment, the judicial balancing tests, and the political process working through elections would provide the checks and balances to correct any over-enthusiasms.
When strict constructionism first made a modern appearance, one scholar suggested that if the Founding Fathers intended that degree of rigidity, they probably would have required a constitutional convention to be called after every census to propose amendments!