Could these people be elected President?

[Emsworth]

Previously renounced U.S. citizenship, but has since become a citizen again.

No.  The US will nto allow you to gain ctizenship after you renounce it.  Once you renounce your status as a US citizen, you can never be one again.

Suppose Congress passes a law stating that you can.  Is their a constitutional bar?

In that case you would probably count as a naturalized citizen.

Why?  You are a citizen by birth, a "natural born citizen," you are currently a citizen, and you've been here for 14 years.  I'm not seeing a constitutional bar.

I would argue that a natural citizen is one whose citizenship exists by virtue of his natural birth alone. If a person is born in the United States, then renounces his citizenship, but later resumes it, his citizenship is by virtue of his later resumption (which amounts to a naturalization), not by virtue of his birth.