Soph good luck getting on top of this. I'd be really interested to hear what the mouth infection turns out to be (get the dentist or your doc to do both viral and bacterial swabs if you can) as it might help solve the mystery.

...My DD (who was 8 at the time) developed similar - horrible horrible blistering ulcers in her mouth only and we had every theory from first episode of cold sore herpes, thrush, trench mouth...and in the end the viral pathology came back that she had chickenpox active in her system (and she had been vaccinated for it too) (!) Which may not relate to your situation at all but thought I'd post as it can be quite hard to tell what the problem is.