I was diagnosed with TOS and started doing PT last year. While I've seen an improvement in my symptoms I am by no means back to normal. I continue to do exercises to help my posture and try to keep my shoulders strong.

When I do front/lateral arm raises I have to concentrate on my shoulder blades to keep them down and back the whole time, otherwise I flare up. I use a light weight, I'm up to 2 sets of ten with 5lbs but I started with only 3 lbs last year. If I feel worse, I stop for a couple weeks until my muscles aren't so tight and concentrate mainly on stretching. I also do YTML's (google it) again with a light weight. I'm up to 2lbs thinking of upping it to 3lbs.

If I'm feeling bad, I can normally still do pelvic tilts/leg raises for abdominals so I feel like I'm at least doing something. Also, I take a walk with the dog. Running is out of the question, (I ran cross county/track back in the day)mainly because I can't always take full, deep breaths. Not that it hurts, like it does with you, just that my ribs get tight and don't want to expand.

I try to focus on what I CAN do instead of what I can't do anymore. I agree with you though, the whole thing is a bit of a paradox. In order to improve your posture/symptoms you need to strengthen your muscles but while attempting to strengthen your muscles your symptoms worsen.