Aww, that's too bad, but I don't think it is entirely your fault because they were fine until you left, so how could you have known it would happen?
I think what you may need to do is introduce all the mice on neutral territory. They might be acting territorial. I think I would get them acquainted in the bathtub (with something blocking the drain) or a large bin. Then I would clean the cage that they will be living in together really well so that they can't smell their own scent on it. Then I would try adding all the mice into that cage and see how things go. I would do it at a time when you know you'll be hanging around and can intervene if necessary. I would probably not add the injured mouse until she has healed up, though. I hope she is better now.
By the way, if you think her wounds might be getting infected, here's what I do because it has worked for me in the past: I would fill up the bathtub with warm water (like as warm as you would shower with). Test the temperature with your wrist; you don't want it to be too hot, but it should be warm enough. The idea is to simulate a fever, which is the body's way of killing infection by "baking" it - but really what you're doing is giving the mouse a warm soak for several minutes. I would let the mouse swim around in the tub for maybe ten minutes. Watch her closely to ensure that she keeps her head above the water and let her rest on your hand for a bit (while still in the water) if she gets tired. Mice don't really like water, but they are strong swimmers and should be able to handle it, however, if you have a very sickly mouse, she might not be strong enough to swim, and you have to be especially careful about that. If you see her blowing bubbles, immediately take her out and dab the water away from her nose and mouth. If she can't swim, you can either have her sit in your hand in the water or let enough water out of the tub so she can touch the bottom.
After her bath, take her out and dry her with a tissue or paper towel. I just loosely wrap the mouse in a tissue or paper towel initially to get most of the water out of the coat and then use a dry one to wipe and dab away the rest as much as I can. She should be able to groom her fur to dry it out and fluff it up the rest of the way. It's important to keep her warm while she's wet, so it's a good idea to do this with the bathroom door closed and don't leave the bathroom until she's done a fair amount of grooming. When you leave, just keep her with you so she can be warmed by your body heat if it's cold in your house. What works well for my me is to wear a hoodie with the hood up and put the mouse inside the hoodie on my shoulder.
I would do the soak once a day or so until the swelling goes down.
Maybe it sounds crazy to give a mouse a bath, but I've done this before with a male mouse whose shoulders were so swollen from fighting with his brothers that I started to wonder if he had tumors (he looked like a little football player). I had him in isolation for days and the swelling didn't go down. Then I tried the warm bath idea, and after a couple of days of baths, the swelling went down and he healed up quite well. Also, when I had a really bad cat bite on my hand and had to be on antibiotics for it, I was instructed to soak the hand in warm water for twenty minutes twice daily for about a week during the antibiotic treatment. I know that mice can be given antibiotics for respiratory infections, but I haven't tried it with injuries because the baths alone seem to work.
Good luck with her, and I hope this information is helpful!