I guess there's no way of telling if one frog is doing all the pooping and the other isn't? You might want to try quarantining if you have an extra tank.

The hanging back legs sound pretty bad, so yes, maybe better take her to the vet.

Safe drinking water for humans is not always quite as safe for frogs, I've read. We can tolerate more chlorine than they can apparently. And yeah, that water treatment stuff sounds fine.