I would argue that, if a power is vested in Congress, then the President may not exercise that same power, by ratifying a treaty or otherwise. Thus, a treaty may not prohibit the slave trade (since such a prohibition is a regulation of international commerce).
I agree that this rule, if accepted, would resolve our slave-trade puzzle. Even so, it leaves much to be desired--at least by itself, and without more.
In the first place, it is somewhat odd to suppose that a treaty may do
only and
precisely those things that are placed beyond the reach of Congress by the enumerated powers scheme. May Congress adopt a treaty that provides (to use some purely random examples, of course) for a federal remedy for violence against women, or perhaps federal punishment for carrying a gun to school?
But let's set that aside. This construction (by itself) also seems to leave First Amendment freedoms at the mercy of treaty-makers. Suppose that, as a condition to the Louisiana Purchase, Napoleon insisted on a treaty provision barring any criticism of the French government (or, worse still, French wine?). In our fantasy world, President Jefferson and a sufficient supermajority in the Senate reluctantly agree. Because Congress has no authority, under the enumerated powers scheme, to interfere with speech and press within the various states, the president and the Senate may therefore (yes,
therefore) adopt a treaty to do precisely that. And because the First Amendment commands only that "Congress shall make no law...," that treaty would be valid. (Ironically, the Article IV Property Clause might mean that freedom of speech and press were secure within the Louisiana territory itself. Go West, young man, indeed.)
In short, your rule makes sense out of the slave trade provisions. But it is not sufficient to make sense out of the broader constitutional structure.