I think 'nearly' leaves some wiggle room. It's probably not a bad idea to be able to get the cover off quick when it's time to quench.

I don't know if heat would drive the O2 out, but it should be consumed when the little bit of it in there gets burned up with the charcoal. I'd think the pressure build would be potentially dangerous if the crucible was air tight. I think Ken's pop he mentioned was an inadvertent air tight seal.