Therapy dog is just an obedient, calm, confident dog. It's just a certification that says your dog can go places like shelters, schools, retirement homes, ect. and make people feel better with its presence. I don't want to down play it but it's really something a well behaved, highly obedient dog can do. It is difficult to do it when the dog is young though so props to OP for doing it that young.
I'm pretty sure you cannot do the test with a prong collar on. Or pretty much any correction collar. It is usually done on a flat collar right along the CGC. Not sure who gave you the information that with a prong they would need a perfect score but its is pretty much like an AKC trial...no prongs allowed.