Since you brought up a Jenova analogy, I'll use another FFVII metaphor: The Lifestream. When a person dies, their life energy is recycled through the planet, until it gives birth to a new entity. Similarly, when a sorceress dies, their powers are recycled to a new vessel (albeit one already alive). It's not a case of the powers keeping the sorceress alive, it's a case of as soon as the sorceress dies, her powers, like her life energy, are recycled into the system automatically. A sorceress can choose to give up her powers before death, but can not die without passing her powers on, for death itself will pass the powers on. When Edea meets Ultimecia, she is saying that she is looking for someone to pass her powers on to so that she can "die in peace". Perhaps death without passing on ones powers first is possible, but not very pleasant.
In any event, since the preservation of life by not passing on powers is a direct result of being a Sorceress, I would say that the Ultimania stating that a Sorceress's lifespan is the same as a normal person's rules that possibility out.
Also, there is no need to keep Adel alive indefinitely. Laguna knew full well that Adel was not the only sorceress: He knew Edea personally after all, and never sought out to seal her away. He only tried to seal away Rinoa because Ultimecia had the ability to possess her. Adel was the one that they were fighting, and it seems to me that the seal was just to stop the threat from Adel herself, not to stop the power of the sorceress in general.
