Hi, this is a great question. You have 3 professional opinions already: here is one more...
It is unclear whether the ATFL is torn based on the imaging and the exam. One option is a repeat MRI
, possibly with contrast to try to confirm the ligament integrity. If you can find a center with MSK ultrasound
proficiency, that is another option for diagnosis.
With regards to treatment, that is a harder question, because we do not know if there is a tear or complete tear. However, Brostrom or modified Brostrom is usually done in patients that have recurrent sprains after rehab and return to play. At 12 yo I would favor conservative treatment and potentially follow up imaging with rehab instead of surgery.
Best wishes for your health,,,,