Just do the same as with Chapter 13 Verse 2, and give us a prompt asking if we want to do the Episode that fits into the current part of the game we're at.
This, pretty much. But I think it would be better bring the option up when you are camping (and the story is at that point). "Where could **THAT** character be", and then you choose to play that episode. And when playing that episode when you reach saves, you could switch back to Noctis)