I agree that Kessandriah is an underused character. I'm wondering if she and Skullmar might make a comeback in future Heroquest expansions. In our heroquest homebrew meta, she takes also a far greater place.
It's true that the "one page rule" (even if i understand it), requires limiting our number of ideas to put in the quest, wich can be frustrating ! : )
But I particularly like the idea that you present here, which puts even more pressure on the adventurers. The design of the quest then corresponds to a real, big boss fight which may require reworking the entire expansion of the return of Witch Lord so as not to have this quest overshadow it. However, if this is done, the result could be very, very badass.