I would highly suggest getting a few other
chickens to introduce along with the hen (assuming the other girl doesn't come back), and preferably from the same flock as the new hen came from originally so they already know each other. You're running into this gang fighting issue because all of the fighting is focused on the one hen, instead of spread out around multiples. Another thing is that it's an established flock on their home turf, so while there's not really anything you can do about it, it may help break down what's going on. Believe it or not,
chickens are pretty territorial (at least in my
experience regarding introducing new chickens). If you introduce an older hen to a flock of younger chickens on the younger chickens turf, the older hen will most likely back down because the younger ones are way more confidant and she's in their space. If you introduce them in her pen, specifically if there are more older chickens, she will fight back stronger. They are more confidant and assured in their own pen. If you put the new hen in a doggy crate inside the pen for several days to let them adjust to her, it might tone down the actual injuries and help them adjust faster, but you may risk injuries from the cage itself as they attempt to fight through it. On a good note, she is coming back to roost at night, so that's quite the accomplishment! I've had to go through weeks of manually putting new pullets to bed during the introduction period. Maybe try giving them scratch grain and food scraps for a while, if you don't already do that. That way it'll distract the flock in the excitement of the new food, and it won't be in a tight feeder situation that is simply begging for fighting. Fun food that's spread throughout the pen is a great way to help the flock adjust to the new hen without the same focus of her being the only interesting thing in the pen. Bringing it back to my initial thought, I would highly suggest getting more hens to introduce with her, and maybe trim their wings before putting them in so that they can't fly out. It's a nice escape method, but it's just delaying the inevitable fighting and risking her not coming back.
Make sure she has access to food and water, even if that takes adding an extra feeder and waterer for the moment. What breed of older chickens do you have? I know that introduction intensity does vary depending on the aggressiveness of the older chickens. Some introductions are bloody and intense, while others will hardly show sings of flock adjustment.
Just out of curiosity, if your older flock is slowing down, why are you holding on to them? I've experienced hens laying great for four or five years, so it's not that I celebrate constant butchering, but it is a question. If they are being bullies, specifically if they aren't laying, you might want to investigate replacing all of your hens. Then you can avoid the introduction phase altogether.
Good luck with whatever you decide! I know introductions can be super stressful!