Hi and welcome to the Chiari forum.
Since I see you are in the UK, The Ann ConRoy Trust is where you need to look for help. here is the link
Since it is located in her mid thoracic spine, did she have an injury there? I would also avoid Chiropractors until you see a specialist....and yes her bladder issues could very well be related to the syrinx.Depends exactly where it is. Did they indicate how they feel she developed the syrinx?
I would also suggest she refrain from her Cheerleading....it could aggravate the condition.
Not knowing all you can request in the UK regarding medical records, (we get copies of ALL tests including MRI on disk in US) that could help you and other Drs understand this better....but a specialist with Chiari and Syringomyelia is best.