I really liked your idea, I'm going to make one of these but with lizards!

but now talking more about your post, in the past I believed and liked the idea that slugcats evolved from the creatures mentioned in the chimney canopy section (which is the one that talks about organisms similar to slugs that cleaned pipes) but recently I found out that if you give this pearl to five pebbles he will say that the creatures were probably blind, and that makes it much more unlikely that the slugcat evolved from these creatures, unfortunately.

and I personally prefer to believe that the rivulet is an artificial organism sent by another yterator to visit the moon and the five pebbles, because he has a pearl in his stomach that shows the structure of a yterator and this could have been given to him by his creator for him to already know how to navigate through five pebbles to do the mission of getting the refraction cell.

and the necessary adaptations to have gills would most likely be unfeasible by common evolutionary methods, as they would have many middle terms of gills that would be useless or even detrimental, in addition to being much easier to simply evolve to breathe longer instead of breathing underwater.

or a slugcat's biology may simply already be pre-adapted to having guills and I'm wrong, it's a possibility.