I have experienced a lot of the same things while teaching new divers. If someone can’t clear a mask, they won’t get certified. If they can do it, but struggle a bit, then I may consider giving them the Scuba Diver (not Open Water Diver) rating which requires them to dive with a dive master or instructor. Open Water Divers must be comfortable with mask flooding and clearing, plus complete mask removal. I recommend that new divers, that have complications with this skill, should practice in a pool or large bath tub...by breathing through a snorkel without holding their nose.