This is my understanding of the situation:
Jericho Locks Canal - Sere Palace:
[Choice to send him into uncharted waters, or let him join Railway board]
First choice: Wandering Gondolier leaves you, comes back later as Hell-Scarred Gondolier.
Second choice: Wandering Gondolier joins you on Railway board.
[Anytime after this, at Jericho Locks]
If first choice picked, you can change Hell-Scarred Gondolier into Wandering Gondolier.
[Marigold Station, after advancing further with Railway]
If second choice picked, you can change Wandering Gondolier to Hell-Scarred Gondolier.
You can change between Hell-Scarred and Wandering anytime, and any number of times. With the only caveat that to change Wandering to Hell-Scarred, you need to further advance your Railway until Marigold station, where the option becomes available.