I believe supers do fine at 50% humidity, give or take a bit. In most regions of the country, that is the average humidity level in the air. It's drier in the deserts and wetter close to the Great Lakes, southern swamps and Washington state. Depends on where you live.
As for needing air, if you open the door to the fridge once a day (probably once a week, to tell the truth), the eggs or larvae will get enough fresh air exchange. The larvae aren't respiring enough, nor will you have enough of them, to consume 19 cubic feet of air in a day. As a comparison, I know of snake breeders who incubate eggs in sealed Tupperware containers, opening them once daily to let in fresh air, and this is for a clutch of 25 developing snakes. Snakes use a lot more O2 than superworms.