In responce to evening cross
I myself don't think that your last point could be possible. I say this because I always assumed that sephy was being manipulated by jenova (my main point of this belife is in the city of the centra, just after you beat jenova:
Sephiroth: Because you are...
(lots of flashy noisy dissapeary stuff)
Jenova: ...a puppet
this for me suggests that the only one in control here is jenova) and jenova's plan is to gain access to the lifestream and suck the planet dry so she proberly knew the destructive power. and anyway sephiroth say's it may destroy the planet so i think he knew the risks.
I might be wrong though so I dunno seems stupid of jenova to risk herself like that so she might of not known.
Meh