Let's don't, really. You know what I'm saying. You're trying to parse your way out of it. But, since you will attempt that anyway, let's take it on.
"To. . .regulate, or prohibit". Hmmmm. "Regulate" includes "prohibit, so that's redundant.
Prohibit. To forbid by law; to prevent; - not synonymous with "regulate." Black's Law Dictionary, 6 Edition
The police power in the US is very specific. Rights may be regulated but cannot be prohibited by that regulation.
"any Substance, Item or Activity". Well, that's "commerce".
I scrape some horn-worm droppings off my tomato plants, that's a substance. I bake a clay pot from scrapings from a clay bank to decorate my mantle, that's an item. I dig a hole in my back yard, that's an activity. None of which have anything to do with commerce.
"to the People". That would be "among".
No, that would be "to", "the People" being the objects of the predicate. ". . . considered to be detrimental among the People of the United States." Makes no sense, does it.
"of the United States". That's "the several states".
Since the 14th amendment, it is "the people of the United States" because the 14th amendment awards citizenship at the federal level instead of the state level. Prior to that, it was "among the several states".
Doesn't change the salient meaning, however, either way.
So now, what do we have? "The Congress shall have the power to regulate commerce among the several states" There you go.
Excuse any typographical errors; I find it hard to type while laughing so hard.
The point you are trying to avoid is, why didn't the framers, if they wanted Congress to exercise prohibitionary power over individual actions and possessions, state it so up front, in plain words in the Constitution. They certainly could have; each of each branch's powers are clearly defined n plain words.