Hmm well from the pictures the fish doesn't look too happy. It's fins are not erect like the other mollies, they look rather clamped. Plus the little fella looks a bit thin compared to her tank mates. In the second and third picture the poo looks almost white which could indicate internal parasites. If she has them then that could very well be the cause of everything. If her immune system is weakened then that would leave her open to other infections, hence the wound on her head.
See if she's eating. Taking food and then spitting it back out is a sure sign that the fish has internal parasites. If you want to, you can float a 1 gallon pail of water in the main tank (to keep it warm) and put meds and an air stone in there (with the fish) to treat her. Tetra parasite guard works ok but splitting the tablets into one gallon portions will be interesting. API general cure works better and it's in powder form so splitting it into separate portions would be easier, the down side is it cost a fair bit more.