Crank gear not seated.

Not really true; if the crank moves back a bit at a time until the rear thrust face is against the main web and cap, then the next blow is all absorbed in the thrust bearing. That is the case you describe with your hand hard between the crank and the wall, that will hurt as you say.

I would not ever advise anyone on an open internet forum to do something that MAY damage things without adequate cautions, qualifiers, etc. In this case, a 16 oz hammer if used hard enough or enough times will move the crank back to the thrust face contact. I don't want to try to advise anyone on how many times or how soft the use of a 16 oz hammer needs to be to not damage anything.