You may want to cage her inside the coop or run - use a see-but-don't-touch approach. You don't mention your location, so consideration of your current weather conditions would be needed as well.
She's not been part of the flock for some time now, so she is an interloper/stranger. Doesn't sound like she has a lot of strength to defend herself either, so a slow introduction is probably needed.
You can try removing the bullies, but...once you put them back, they probably will take up where they left off. Chickens are not always nice to each other.
Do you have photos of your setup - how much space do you have? Is fencing off a small section doable in your run? If it is, once she is next to the flock for a few days, take one of your most docile friendly hens and put them together - monitor them. Let her have a buddy for short periods of time. You want her to have interactions, but you don't want to stress the "buddy" with long separations - make sense?
Sometimes you have to try quite few different things. Having patience is going to be important. One thing may work for me, while it's a disaster for someone else.
Keep us posted and we'll help you try to figure this out.
Here's good article about introducing a single hen to the flock:
http://www.backyardchickens.com/a/introducing-a-single-hen-to-an-existing-flock