To make people leave... put a level limit on it. Once you reach lvl 6 you're no longer allowed inside anymore.
I dislike this suggestion just because it would ruin the 1v1 pvp that I have happen to me there from time to time.
...the idea is not to roleplay moles or stay underground all day long...
It definitely sounds like a good idea, but in my opinion it can be improved further:To protect from making this place yet another battlefield: some level limit, that restricts high-level characters to enter.To protect from suicide-bombers: add a couple of respawning guards there that are checking your inventory and your hands for anything illegal. To make this place more interesting to visit: a simple dungeon with low-level creatures.
You want to help newbies? Shoot them in the face and take their items. That's how they learn best.
Safety is going to be the biggest issue with your nuubland. If the npc guards are weak, they get wiped out and the place will be camped by dedicated playtesters. If they are too strong you end up having a new ncr, unless the npcs work like militia and there's players monitoring it.
All hail Sheriff Wichura.
Checking for dynamites - very good idea, I don't know if possible to implement tho.
... but it's too much time consuming to make all necessary alts, put mercs, camp grid and stuff.