Hi, I'm new. I just wanted to chime in on the speculation! I definitely have a love/hate relationship with the wait between books.
Regarding Listeners/Parshendi/Parshmen: I think it's probably accurate that the Parshmen have to be outside to transform. However, I also think it's possible that they were drawn outside by a rhythm or other unspoken communication. I also agree that Parshendi (ie. Rlain--already bound to a spren) can probably choose to change, while Parshmen cannot.