Contractor installed my solid oak hardwood with no expansion gaps. Repeatedly told my wife, who was overseeing the installation at the time, that once nailed down properly, expansion gaps are not needed.

How big of a problem is this? I'm in NJ/PA/MD area and will go through the typical four seasons. Is this even fixable?

As long as you keep your indoor environment reasonably stable it will not be a problem.

Yeah perfectly stable meaning temp and especially RH, but how easy is that?
Hope they got a warranty in writing.