Cory - Just to clarify -
In your example, assuming physical torture, the stamina would decrease, and they may pass out - but their willpower would be separate.
If the torture was mental, then it would slowly reduce their willpower, and eventually they would fail that resistance. If somehow they continued to resist until their willpower hit 0, then effective their mind shuts down and while they may be awake - they would have suffered a complete mental breakdown.
As for the weakening power, weakening resistance, leading to a potential downwards spiral... well, isn't that what happens? Think about it from a physical standpoint. In battle as you take more and more of a beating your body is less able to deliver damage (frenzies notwithstanding).
jdale -
Yes, some additional DP needs to go into PP, WP, and Stamina regardless of your focused profession (as a general comment), though you will have a base level standard based upon your race so it is still a DP choice and not mandatory. Coming from the HARP background, I also tend to forget that RM costs are significantly more (especially for Arms users buying ranks in PP development). In HARP the costs are a lot closer, and Endurance and the Resistances are favored for every profession, so it would just be a matter of moving PP in there also.
As for the "sturdy, resistant, non-magic-using character".... I disagree. They can still be non-magic-using, but spend DP in PP to cover their resistance. A mage who invests a lot of DP into Body Development (or Endurance) does not automatically have to engage in melee.