I don't remember too much about the Hinterlands, I haven't played through it again in quite a while since I didn't like Storm or P very much until the Hinterlands was finished, due to being impatient to see more of the main cast. I think the implication of the Tapir's salt was that it was specifically pure salt, and not tainted salt. Or perhaps the spirits in the Hinterlands aren't inherently malicious like P mentioned, which is why the salt doesn't affect them?
It certainly would be very interesting if the stipulation was exclusively that living things created by the hotel were hostile. I think Asterion would look very cute, tending to a garden, perhaps this will be addressed in the future by the writers. There certainly isn't any confirmation I'm aware of between a difference of "imported being aggressive" and "native being aggressive", the application of it to guests not being aggressive to Asterion certainly is a good argument.
As for the afterlife, I think that's a question that Hermes could answer. After all, isn't he supposed to frequently visit Hades to deliver messages? Wasn't the letter delivered by him alluded to have been from Poseidon?