[spoiler]....
This is the interaction. It works in arena mode, so I'm pretty sure it would also work if I genned a new world.
Not necessarily. Fort mode has a bunch of extra stuff going on that arena mode doesn't. I don't know what your custom creature is, but I can think of a couple possible examples of things that might break fort mode but not affect arena mode. Like say, hypothetically speaking, transforming an intelligent creature that is a citizen of the player's fort into an unintelligent creature that is
still a citizen of the fort (a relation normally restricted to intelligent creatures)
might break fort mode and create a crash. It might not. Hence the need for testing.
However, that defeats the point of this exercise, which is to put this creature in the existing fort.
The whole point of genning a new world is to identify or eliminate sources of the crash. If your interaction works in a newly genned world, you know the interaction itself is fine. If it doesn't and you are still getting crashes, you know the problem has nothing to do with patching modded RAWs into an existing save. My next suggestion if a new world gen works would be to revert your walrus RAWs to vanilla and try the interaction again. If that crashes, then adding the interaction to an existing save is doing it. If it doesn't, then it is probable that something about your modded creature that differs from the walrus it is replacing is to blame.
Perhaps post the CDI block as well?
While you are at it, post the creature too.
Also, does the crash happen when the world loads, or when the interaction triggers?