"General welfare" is limited by the enumerated powers given to Congress in the Constitution.
Not sure how much clearer it needs to be.
You cannot make that claim: the meaning of the term is not defined by the Constitution, and in the quotes you posted, neither Madison nor Hamilton clarified the its exact meaning. At most they talked about its scope, as they saw it -- and for what it's worth, Hamilton and Madison disagreed on that point.
If we look at it using common-sense, the term "general Welfare of the United States" is probably best interpreted as "things that need to get done that are
not explicitly enumerated in the Constitution." Things like allocating money for the Louisiana Purchase, say -- also not explicitly among the enumerated powers. But it's easy to argue that it promoted "the general Welfare."
The scope is limited in one sense: money raised to further "the general Welfare" must be done to the benefit of "the United States," as opposed to a particular state, or to benefit specific individuals. It can be argued that money to fund Kudlow's idea falls under that clause; and in
Butler the USSC ruled that Congress gets to decide what the phrase actually means.
I really don't think you have a valid argument.