Should you be prepared to put her down? I would say it is good to be prepared, yes. Because it sounds like it's not entirely clear what the cause is, the problem could turn worse than what it already is. What if she gets put in worse pain? It's good to set boundaries of how much pain and struggle is too much; there's no right answer here, your judgment is what's important. The time I would say definitely put her down, if no other time, is if it becomes clear she will not make it. Just keep in mind that chickens hide pain well. But don't lose hope! She could pull through and be fine. It's just important to keep it in your mind and be ready for IF it comes to it.
Can it spread to the rest of your flock? I am not super knowledgeable of chicken diseases, so I don't want to make any assumptions that turn out to be wrong, especially since it's unclear what it may be. I have, however, done research on bumblefoot, a staph infection, and have read that it is not contagious. So going by that same logic, a staph infection like that would not be contagious. However, something happened to your chicken to cause this. If it is some external factor, it could also affect your other chickens. It could also just be something having to do with something going wrong with the individial chicken's body and won't affect the others whatsoever. You may want to keep an eye on your other birds to be safe, at least until it becomes clearer what it could be. Hopefully someone more knowledgeable on this stuff will reply and offer better info.
Will she be picked on? If she has some kind of illness or disability, she will likely be targeted and picked on a bit more. That's not a given, but it seems to just be the nature of chickens.
I'm really sorry you and your girl are going through this. I hope she's okay and you figure out what it is
you're doing a great job, she's very lucky to have you
