Mystery 12 weeks cough and sinusitis in 9 y.o. daughter

My daughter (9 years old) has been suffering with a cough for 12 weeks. We think it started with a virus or bacterial infection and initially seemed like a typical cold/chest/sinus infection.

After about 2 weeks she was given a course of antibiotics (amoxicillin) and she had about 10 days off her normal activities (she does swimming training 4 times a week, does athletics, and football/soccer). She felt a lot better and the cough had nearly gone, so we resumed her normal activities, but then the cough got progressively worse. She was producing a lot of mucus, and coughing up blobs (especially in the morning, so presumably from post nasal drip). Some nights she'd be coughing for hours! We started using antihistamines at this point, in case she'd developed some allergy.

At about week 9 she came home from school shivery and with a mild fever (37.7 C) so we took her to an emergency appointment. Her temperature returned to normal, all other signs were OK, and the X-Ray showed no signs of chest infection. Doctor saw no issues in ears but noted how wet and mucus-filled her nose was. He said the sinuses must be inflamed and causing a post-nasal drip. He gave us corticosteroid nasal spray (mometasone), and another round of antibiotics just in case (penicillin).

2 weeks on and the mucus still wasn't going and the cough is/was severely affecting her activities (especially swimming), not to mention the impact on the whole household of the constant coughing.

For the past week we've started saline flushes every day, which has been removing amazing amounts of mucus from the sinuses. This has had a positive impact on the coughing (I suspect because there's less mucus causing the post-nasal drip), but the mucus just keeps coming. Every day there's as much as the day before! We've just switched to fluticasone nasal spray in the hope that's more effective in the coming days.

We've got an appointment with ENT consultant in a couple of weeks but has anyone seen anything like this before?