I've run into this more than once and have seen posts about it, but it is rather annoying to make a new character get through the intro only to be trapped in the cage in the square and the npcs are dead. Would it be possible to make them immortal so this doesn't happen so easily or often whenever there is a brawl in the square?
Additionally, disable their AI as well if you do this.
I think the issue is less that people are murdering them than they are the two NPCs that still exist in one of the more trafficked areas of the server so DMs will tend to use them when they need a nearby NPCs.
Maybe keep the NPCs there but add some kind of backup clickable that lets you join the city if they're gone?
Best suggestion, move them into an adjoining chamber in the Mythallar chamber as a receiving portal near their surface portal, then people can pick up their sending stone on their way out before they leave as the clerk invariably points them to pick one up anyway.
This leaves the Square free for dangerous antics, and people won't be struck by destructive AoE spells from the Warder, and the awkwardness of players that haven't set their spawn point yet is dulled somewhat if they aren't repeatedly showing up in the Square as a new arrival when they aren't.
^ this..
Is probably the best solution.