Who else has suffered or does currently suffer from repetitive strain injury caused by balloon modelling? Has it ruined your career? Can it be treated? Any advice to someone who has been suffering from the condition for about two years.

I'm not sure how do you define "injury".
When your hands are numb in the morning, is this consider "injury" ?  I do have this problem, then I was advised to put hands in warm water before I go to bed, give a little massage, about 5-10 min. it does help a little.

I suffered from tendonitus once as I twisted for six hours solid without a rest. I learned by my mistake and now take regular breaks.