I'm not 100% sure how this suggested change works. How would the various rolls determine which roll is compared against which? It sounds like it will at least solve the 'less defense can be better problem' but I would need to understand it a bit better to gauge if it causes new problems.
@Mr.rodent While I get what you are saying I feel the need to point out that your plan has some downsides that I believe may very well make the plan nonviable:
-All signs thus far indicate that to create and replace stuff in our dungeon we need to spend mana. This mana is the same stuff that we would be trying to get from the adventurers who we would be enticing in... There is a clear conflict there, if the adventurers break/steal too much stuff without us being able to harvest enough mana from them to replace said stuff we'll lose mana from having them in our dungeon, not gain it.
-We can either kill the adventurers or not. Leaving aside any serious defenses around our core we would still need to kill adventurers to gain kill-mana from them. As you already indicated adventurers are unlikely to be happy to die. To maintain friendly relations we would need to keep a pretty tricky balancing act up.
-By spending mana on constructing a 'friendly' zone in our dungeon we would be taking away mana from our core defenses when compared to a fully fortified dungeon. We will be weaker against dedicated attack because of doing this. Or at least until it pays of with sufficient mana to pay for itself (if that works that is). Remember that if the locals like us that anyone who doesn't like the locals will also likely not like us, we will still have enemies even if we are beneficial.
-If we start being generous and help the locals build up strength and wealth then we need to keep in mind that they could likely amass more power than we have. Which in turn means that we are to some degree at their mercy. We would likely be the weaker party in the arrangement. And being in another party's power is IMO not desirable.