Does my Molly have columnaris?

You cannot keep doing the extreme changes, the fish will go into shock. The other fish were stressed since they got in the tank so really your Molly was getting beat on by the other fish. They get really aggressive in tanks. You may want to consider a bigger aquarium and keep the water the same as the way you've got it now so you won't kill the fish. The Molly is fine it was just attacked but if you keep changing that water from one extreme to the next, it won't be.