Tooth extractions normally heal up within a couple of weeks, maximum. If your cat is retaining fluid, it is due to the steroid, the dexamethasone, so a diuretic will only mask the symptoms caused by the meds. It sounds like there is an infection in the sinus, due to the opening left by removing the canine. This likely accounts for the whistling sound. Since there is a discharge from the left nostril, have the vet send this to a pathology lab for a culture and sensitivity to determine what is causing the discharge and what will treat it. Have him check also for fungi.