acceptable abdominal exercises?
 

that won't worsen TOS?

I do trunk rotations, bridges, crunches with the head supported by hands and leg raises (one leg at a time so that hip flexors are not engaged).
Though at my worst I could only do abdominal breathing.
Usually takes me a few weeks to find out if an exercise is safe to do. I add them one at a time starting with just a few reps and see how the body reacts.