But your argument seems to be that she wants to be defeated. If that's true, her actions belie her intentions. If anything she's trying her hardest to kill Squall and his allies without pulling any punches. She fights her own destruction tooth and nail.

And in any event her goal seems to be omnipotence as opposed to immortality. Although compressing time might fufill the second item, what she really seems to be after is power.

EDIT: And I know longevity of sorceresses is a big issue for U=R theorists because Rinoa cannot become Ultimecia if she has a normal lifespan. I believe the FFVIII Ultimania laid that possibility to rest by stating that sorceresses have normal lifespans.