Well, we agree on your last post. Congress can certainly write a law that includes the unborn as persons.
Yes, Congress can write such a law.
And the Supreme Court can also decide, as a matter of constitutional law, that the “persons” protected by the Constitution are human beings, and that human beings come into existence at the time of conception and become persons.
And no, Congress could NOT pass a law (and have it stand) that said that persons under the age of 3 were not persons for the purposes of the Constitution. The Supreme Court would have the authority to strike down such a law.