1. you can wash it to avoid infections. I use warm water/epsom salt mix, which does not sting, and cleans very well. I've used this method on sores on previous cavys, and the sores went away within 1-3 days

A soft cloth, paper towel, is your best bet. Let it dry itself - to avoid towel particles from getting into it.
2. infection will show easily: puss, pain, swelling, and a "rotting" smell.
3. check it every day.
4. If the fighting isn't reoccuring, and they do get along with very little to no scats, they can be together. If one is bullying, you may need to separate and try reintroducing 1-4 weeks later(NOT in cages, but in no-one's territory!). If it still does not work, they need to be kept separate for good. Some cavys are just meanies.