The only alternative I can see is that in the absense of human
: intervention, as her senility became more apparent, the High Councel would
: have developed a plan to create a replacement.
Good point, the way I tried to explain this part, at least in the story, was that as the Empire grew beneath the queen she required more assistance in maintaining the empire, and this was supplemented by the the birth of the bureuacracy and the military as well as incorporating various slave races to perform functions she could no longer do, i.e., the S'Pht could process data while the enforcers kept the fighters in line. I think it's fair to say she may have thought she'd have "solved" the Jjaro puzzles and ascended before she'd have ever become so decrepit, so it is possible that no arrangements were made for an immediate successor. And a successor would be difficult to find, appointed by the council/bureaucracy or otherwise, especially if the queen herself had not developed one.
Even so, the Empire seemed to function well enough without her direct control in its latter days; there is competent leadership in the Empire though I'd wager these individuals would still be tied to the queen's will either by some direct implementation on her part or a fierce loyaly on theirs. It would have been in the queen's best interest to make the Empire self-sufficient as it grew otherwise it would collapse as you predicted. In fact, were it not for a few passing references in some terminals we as the player would know nothing of the queen at all; the Pfhor themselves were running the day to day business of the empire and doing fairly well at it.
In the story the Queen and the Empire fall from power at nearly the same time, and it may very well have been the war that threw the Queen into senility in the first place...
