Ultimecia's Power

[Sephiroth]

Ultimecia has no knowledge of what has happened because she is not really immortal. She can control the powers that were given to Edea by Future Ultimecia but her consciousness still dies. Ultimecia is only aware of a few events but does not actually know the full event chain which is the very reason why she wonders about stuff her future self has already lived through. She does not event know what Edea knows because when she is in control of her body Ultimecia cannot make use of Edea's memories. She is stuck in an event cycle that is doomed to fail. A cycle she does not want to be repeated but she cannot do anything about it.

-Ultimecia is conscious of a time period in which her powers are inside of the body of someone else. She experiences that time and makes it her present this way. Ultimecia has also full use of the powers a witch possesses, no matter what Edea can do now or not. Ellone is supposed to be used by her because she wants to project her consciousness even further into the past and so combine Past, Present and Future by making it her own present. Technically something that could never be fully done but let's ignore that. She never succeeds to fully kill everything anyway.
-Ultimecia can control other people this way, normally by making them unconscious and projecting her own consciousness and self to animate this body.
- Sometimes the game makes no difference and speaks like she just corrupts people in terms of "they are not themselves" in a metaphorical way were you just have changed so much and that would also be possible because of Ultimecia's power which would mean people are actually conscious but are corrupted and bound to Ultimecia's command while she actually watches - most of the time she actually acts herself though, still she can also use that trance like she did with Rinoa. Her power to control can be broken as well but it cannot be simply gotten rid of which is why Edea feared that Ultimecia would use her again and why Edea had enough time to build the Balamb Garden before Ultimecia controlled her and knew what exactly was going on. Ultimecia transferred herself into Rinoa though which is why she didn't use Edea anymore. That does not mean she is limited to one person to possess though as she also wants Ellone.
- She can materialize thoughts such as Griever which means she can partly control reality in another way. Whether she has that from other Witches at that point or not is not really relevant. It even is a bit similiar to Draw as G.F.s are an energetic and psychic lifeform unit but it was never explained what exactly is the nature of a G.F. and how they fully start to exist so it might be a different form of magic like the living matter spell that she uses with Edea's body.

And Edea is a witch all the time. Edea's body had the power of two witches. The one she got when she was 5 and Ultimecia's when she was like in her 20s and Ultimecia was forced to give her power away after Squall and the others killed her. You cannot actually decide to not give your power away. Your body forces you to do so. That is explained in the Ultimania. In Ultimecia's case it was just convenient.