I haven't dealt with sour crop before but I've heard giving plain yogurt with the electrolytes still in it could help, and even if it's not sour crop it shouldn't hurt her. If it's impacted crop, messaging it is good and even getting her to drink some olive oil or coconut oil could help loosen...
It sounds like you're doing good, I would keep an eye on it and make sure it doesn't start to look infected. Keep it clean and do your best to not let her peck at her injury. If it's deep you could us NewSkin to help it heal faster. I don't think she'll need aspirin personally, but I don't know...
It weakens them, weakens their immune system and makes them anemic, which can make them susceptible to other infections and diseases. But otherwise I don't really know what else they may cause.
Make sure they're getting enough niacin. It's very important for ducks and there isn't much in chick feed. Brewer's Yeast has some in it, and you can supplement it, too. I don't know if there was an underlying cause, though. I'm gonna tag @Miss Lydia
Also, to help prevent more parasites like mites and lice, letting them dust bathe is great! If there isn't dirt around, you can make an area with sand, charcoal, dirt, and DE.
I'm glad to hear that! Awe poor thing, that's always a hard part about isolation. One time, I had a girl who couldn't go back into either coop I had at the time, so she became our loner, and she loved it. She just free ranged all the time and then came back to her pen at nighttime. As long as...
It doesn't look too bad. If you feel more comfortable you could put the hobbles on him/her, but otherwise I would just keep a close eye on 'em. Also, you can put electrolytes in their water for added vitamins to strengthen them.:thumbsup
Good luck!