Yes, that’s very much the point. If the heroes ignore the Husks in order to focus on other threats, the Husks will eventually take care of themselves, but at great cost to the heroes. But if the heroes spare some time and effort to defeat the Husks before they self-destruct, they’ll be in much better shape.
Yes, it does. My thinking is that the bonus is what the heroes get out of the bargain, and the Hinder and/or Attack is what they have to pay for it. As an analogy, I wouldn’t sell you something and then demand the thing right back after you pay for it.
(However, I suppose that one could see it more in terms of a bank loan: the fey loan the heroes power, and then demand it back with interest. Either interpretation is logical; the first is simply the one I’ve chosen to use.)
Hmm, Necrosis. I think his Approach would likely be one of Creator, Dampening, or Leech, and his Archetype could be Inhibitor, Overlord, or—as you mention—Domain or Titan. The latter two likely wouldn’t work for his standard operating procedure, but I think they make sense for this situation. I do like the idea of him becoming some sort of gigantic pollution-monster powered by fey-magic, and his status challenge could even represent the heroes siphoning that energy away from him.