Ok, as I have said before, I am a people nurse, not a doggie nurse, but my experiences with abscesses is that if they are draining on their own, this is good. Sometimes gentle irrigation of the abscess with sterile saline through the draining tract can be done by the provider (vet) to loosen/remove any remaining pus. Did the vet rule out a dental issue or dental abscess as the cause? My concern with recurrence would be that the original source of the infection is treated or removed. You said she has a serious respiratory infection. Does he think it came from the sinuses? Would possibly adding a med to keep the sinuses open like a decongestant help avoid reinfection? Hopefully the full course of antibiotics will take care of all of the infection, but an ultrasound of the affected area after the antibiotics are completed could be an easy, noninvasive way to assess if further treatment with antibiotics is needed.
Just tossing out ideas here. 