And your complaints about Promptos clothing are easily explained. When he was knocked off the train he fall in the non snow part. He could have picked some clothes up to fight the winter. If you are going to get into the knitting gritty then explain how the bros put up a full tent and BBQ while being no where near their car haha!
Also Prompto gets knocked off on chapter 11 and isn't seen again until the end of chapter 13. You meet up with Arenaea in tenabrae with Noct then she meets up with Prompto. But what you probably didn't notice was that Arenaea was actually Ardyn, she was never there. That part is only explained in the one last line of the episode.