Welcome Tamsin, you did the right thing going to the maxillofacial surgeon, hopefully a cancer specialist, and going to have an ultra sound, biopsy done. I had a calcium stone in my salivary duct, I didn't know about until I had a PET/CT scan ordered by my ENT, so he took it out while doing my endoscopy, under anesthesia.

Did your GP do any blood work to see of any abnormal levels? The collar bone, and armpit lymph is not a usual place for oral cancer involvement, but anything is possible. Based of your medical issues mentioned in the last paragraph, these may have something to do with your current concern of the mouth ulcer, and may be a systemic condition. Did you see a doctor about these infections, and take any medications? Some meds can cause mouth ulcers, as well as other medical conditions causing them too.

If the thrush was not resolved, which is a fungal infection and easily treated by anti fungal medication, it can go into your blood system via the ulcer, esophagus, stomach, and go throughot the body and become disseminated or invasive candidiasis, which is more difficult to treat, very dangerous, and can be septic too. I would ask your Dr. about this, maybe check for infections, do blood culture, oral culture, and have a chest x-ray too.

Keep vigilant, and hope you find answers. Good luck.