I think I ended up at the same spot as you. Since the tapir god's salt is what lets Storm purify the tree, my last idea was that it would also let him exorcise/cleanse the ex-husband, but no dice. I also have no idea what's up with the slug. What even is the trigger for that? I only got it once, and it seemed like it came from re-visiting the colonel's estate before going to the tree but I'm not certain. And what would the significance of the slug be? afaik, you can only visit the tree on the last day so there's nothing to do with the slug after you get it because there is no "after." Hm.
i've seen 4 different iterations of the slug scene:
- revisiting the colonel state will have P kill it
- revisiting the state and returning the pelt will have P relocate it and get bitten
-doing neither, it will give up before doing anything
-just returning the pelt, and learning of the sleepwalker brand P is protected from it
i might be off about some of the conditions but those were what i took notice