As to who is welcome in Neriak:

I know that I am quite welcome, even in the 3rd gate, and I am Human. In
fact, the guards in 3rd gate are friendlier to me than the ones in the
foreign quarter! I am a human necro who follows Innoruuk, so the other
human necros, I don't know about.

There is one place I am KoS'd in 3rd gate. The ghouls guarding the Hall of
the Dead don't like me very much. So, I go invis to undead to getr past
them. In the Hall proper, I am very welcome and have done a number of
quests, trained, and even bought one of those cool gray necro robes the dark
elves wear.