46erjoe wrote:

If I were to seal every one of these cracks with urethane calk and
thus make the basement airtight, would the radon problem go away?

It might, if there were a vapor barrier under the slab.

Or can radon leach through concrete block and floors?

Sure, but you might put poly film on the floor with carpet over that
and poly film on the walls with foamboard over that.

Nick