Best of luck Pura, I hope you find something really nice!
Hmmmm, oxygen is definitely essential for respiration and therefore growth, but I know that a bunch of people run sealed rooms (including myself) and I have not had plants full on die from lack of oxygen. 47% humidity is too low in my book, but I know other growers who rock 40-50% their whole cycle and get good results so it seems to be just a matter of balancing that with your feeding input to avoid stress. Powdery Mildew shouldn't be able to grow on inert media or soil, it infects the epidermis of plant tissue and can only germinate in 95% humidity so I doubt that is what you were seeing on your soil with half the needed humidity for PM to thrive. What it does sound like, and I'm sorry I can't throw out more exact possibilities here, is that you may have a soil borne pathogen of some kind and that is preventing the plant from growing properly (would explain why the same genetics in flowering are not having problems if they have not been infected/inoculated). I would pull a plant or two out of their pots and inspect the root system, since an unhealthy root system can cause symtoms just as you described (lower leaf lose followed by rapid plant death because there is limited/no water and nutrient uptake by the roots), or since you said you recently transplanted you might have gotten a good look at the root system already, and can verify that it was not white and filling the pot. Either way, I would make sure to avoid any potential cross contamination between the plants you say appear healthy in flowering, and the ones having trouble in veg, just incase it is a pathogen. Obviously it could be a dozen other problems, but better safe than sorry until you know a little more about the cause.