"If "people/persons" only refers to citizens of the United States "collectively" (your term), then why did the Founders distinguish between the two elsewhere in the text?"
A Person is an individual regardless of citizenship. "The people" refers to the citizens as individuals. Simple, no?
"As far as the Constitution, as Supreme Law of the Land, applying to every person withing its jurisdiction, that's not penumbra-speak."
So, it DOES protect an invading army in your world....
Shhhhhh. Be nice my friend. This is a person that is still trying to figure out what the word "IS" means. There is a reason we don't enjoy the rights our US Constitution guarantee us while visiting other countries. It's because their governments read their Constitutions and our Constitution the same way we do.
Let him blab.