I agree. It sounds like she is just thin from the stress and not eating well. I got a hen from someone like that and she fattened up with a little TLC, which you are doing for her, anyway. I also think that maybe you are noticing her breastbone more because you are trying to find out what is wrong with her, so was brought to your attention. You are probably handling her a lot more than your other hens, because of her injuries. I bet if you grabbed one of them and poked around, they would be similar.
As far as the joint goes, it just won't heal overnight. My son's horse got into my chicken barn and stepped on one of my roos. His leg swelled up and he didn't use it for a week. I thought it was broken. He would hop around on one leg. I couldn't feel a break, and he got around great, so I let him be. After several days, the swelling went down, and after that, he started putting weight on it.
When we butchered him a few weeks ago, I specifically checked his leg to see if there was an old injury, there wasn't. I should have been able to see something, since it was only a few weeks earlier.
It sounds like her attitude is good, and she is getting better. Unless you just want her to go, it sounds like she will be fine. You are taking excellent care of her!
Shelly