I have not seen the other threads but if they got 'sick' that quickly they had it before they were brought home and could have been made worse by the environment it was brought into. The number one reason for young birds to die quickly is cocci although they can find a hundred ways to kill...
Your vet sounds like mine, always concerned with dust and sinus infections and I can't say she is wrong. Dander and pulverized dung being inhaled will lead to respiratory infections. We have one large coop that we had to add extra ventilation to after battling the same infection outbreak about...