First, I would check your pH of the tad's water. I've gotten this before when doing a water change with water that had a much different pH than the original. Also, I'd suggest using an antibacterial in the water like trisulfa, or tetracycline. This has also worked for me in the past.
First off tadpoles will feed upside down as you describe, they will feed on whatever is held by the natural surface tension of the water, the same orientation can be observed when you sprinkle finely ground food that floats. One good thing to do is after you've fed the tads spray the surface of the water with a spray bottle, so all the flakes sink. Sometimes tads can swallow too much air which gets trapped in the body cavity, causes swimming problems and possibly death.
The twirling swimming motion is not good and usually means the water not optimum quality or the tad is physiologically doomed from improper development. Check the pH, hardness, ammonia, nitrite and nitrate of the tad water, especially if you are using tap water. Local fish store or on line aquarium suppliers will have these water tests for rather cheap. I use RO water with an addition of Black Water Extract to simulate an Iced tea look, this helps keep bacteria counts down and provides slightly acidic pH.