I think I get what you are saying. But I also think that what you describe is very similar to the skills of sport or playing a musical instrument etc. There is only one common denominator, perfect practice makes perfect. What you see as automatic fluency in fluent speakers is the same thing I see with basket ball greats or virtuoso piano players. They practice and they get really good. I don't know why singing is easier for PWS. but stringing together the words and prolonging are just fluency shaping tools with a beat. Its almost like fluency shaping is based on PWS ability to sing.