Ultimecia possessing Edea was because Edea has Ultimecia's powers, Ultimecia has a stronger consciousness than Edea, and Ultimecia is a sorceress, she uses his magic to control Edea and to amplify the machine's power, other reason that gives her enough power to control Seifer, because Seifer was not really loyal to Edea, but to Ultimecia.
Says,who?

Even Pinnoquio can possess Alexander the Great with a proper machine if he manage to clear his memories.
That is the main reason people canīt accept Odine as the main villain.
Their warmonger logic donīt let them accept.
People allways prefer to consider Ulti as the villain.
I am trying in vain to revert this mentality.

we,the pc players had a special advantage
On the pc version one can see the inner lines of the king ghost.Unlike on the Playstation version.
That was what I was talking about.