Hi I am not sure if this is the same situation, as our chicken may have just injured her leg. But it all started about 1 year ago she kept falling over on one side, we heard and thought all the same advice you are getting. And I followed just about all of it. I soaked her, I fed her yogurt, and gave her vitamins. Over time she started to walk a bit better but till this day she is not too steady on what appears to be her left foot. However, she is and continues to be the top hen out 4 of them. She does all else normal including eating, drinking and moving about the yard when they are out free. She does have a hard time climbing up and coming down the "ramp" to the coop which she has to flap her wings to help herself. I'm guessing this is as good as she will get and figure the leg may have been injured at the time and now she has a limp. Hopefully this is the case with your bird too and not a disease. I just thought I would mention it to give you a different spin on it.
Good Luck...
~ Tara