You asked about Congressional power to legislate, not whether 14A grants an abortion right. These are different questions. For example, it seems Congress could plausibly pass a general abortion ban with exceptions for medically necessary abortion under 14A. Moreover, if Congress wants to give itself additional power more explicitly, it could pass an Amendment to give itself authority to legislate on abortion (of course, this requires bipartisan cooperation).

To be clear, you're talking about section 1 of the 14th amendment which reads:

All persons born or naturalized in the United States and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.

First, I thought we were talking about legislation allowing abortion at the federal level. That said, if this court is consistent (not guaranteed), then I still don't agree. This was ratified by states with laws allowing abortion. Therefore an originalist interpretation should not accept that person here refers to the unborn.

But I can see THIS court accepting that section 5 gives Congress the power to pass legislation, defining "person" is a proper legislative act, and therefore the Congressional interpretation of a disputed word should be accepted.

So sure. Congress can try ban abortion nationally, and might succeed. But I doubt that they can legalize it nationally and have that decision hold.

(Separately the amendment process is more complicated. The failure of the Equal Rights Amendment demonstrates that Congress alone cannot create an amendment, even if the public generally supports it.)