It sounds counter-intuitive, but separating chickens from the flock stresses them even more, and when you return them, they get beaten up even worse than before. Give her a single buddy of one of your gentlest hens so that no ganging up can be done, and so that she has a friend when you eventually re-integrate her.
There are a few reasons for feather-picking, a lot of them nutritional. Try adding some vitamin and protein supplements. Take a look at your housing? Is it large enough for your hens? Can a bullied girl find a place to hide? Are there obstacles so she can escape the rest of the flock?
And if she's broody and if you're willing to baby chicks through the winter, by all means let her raise some, but if you live in a colder climate, be aware that you're going to have to baby them. Keep them in an enclosed building with good insulation, probably get rid of any roosters that result, and possibly bring them inside on colder days. You might have to set up a heat lamp.
And if she turns out not to be a good mother, you'll have to keep the chicks inside. In winter. Until they get big enough to heat themselves (and seven week old chicks cooped up in a box? they smeellll.) I'd do it--but I can be a real idiot.
You're welcome, and good luck.
