There is no reason to have a ventilation duct end blindly behind a bathroom mirror. Also when cutting a hole into a mirror, there is no way to put the cut-out back in again and make it look seamless.
Mallory reminds Cheryl that she isn't allergic to shellfish and that it's Pam who is. Pam is actually allergic to soy. However, Mallory has displayed many times that she is not detail-oriented and cares little about her employees. Therefore, it is not unreasonable that she was say that.