You have buyer's remorse. It's not uncommon. Time will usually heal
it (can be up to a year or more).

If you are losing sleep about the unventilated 300 sq ft, drill a hole
in an inaccessible area and stick a sensor down there to measure
humidity. If it's dry you should stop worrying. Or, if you are sure
that the floor joists are not resting on the concrete (how do you know
this?) then drill 2 holes, one each in opposite corners, blow air into
one of the holes, and measure the air coming out the other for mold
spores (test kits are available online) and humidity.

As far as legal options are concerned, it may depend on what state you
live in. In some states, it is required by law that the seller must
declare any remodelling. If the person who sold you the home was the
owner when the remodelling was done, and they didn't notify you, then
you have a legal option.

EJ