Reading the Seven Seals with the thought of Giant Robots in mind, you could make a hell of a giant robot with these things.
A giant robot might make a superb Accretion foci though..
I granted that she would be fine against normal people. If we want to pass all social responsibility to her though; she needs to be actually very skilled, and not just at convincing people. I don't think Gisena wouldn't provide much marginal utility compared to a normal person if you account for her "winning personality"; she's likely terrible at leading people thanks to that. Having her actually administrate things might be even worse.
You are making a ton of assumptions that make no sense. Gesena is superhumanly inteligent, good at socializing, has superhumanly amazing looks, has years of service as one of de facto leaders of her country and so on. She might be not perfectly perfect at any social ever, obviously, but she offers a whole lot, and that's a lot we will need thanks to our Curse. Meanwhile, Cat offers nothing in comparison as far as social is concerned, forcing us to take our Curse head on.
But we have some guarantee against Apocryphal Curse in the immediate- power. Just immediately using the Seals on Catherine boosts her capabilities tremendously and makes her the best fighter by far, without taking into account a possible Soul Evocation. Magic has unknown relevancy in this world; the monsters seem biological, but it might be something like "scientific" like Eva angels or Getter rays bullshit. Thanks to Hunger and Forebear's, we likley can progress fast enough for have some mitigation soon-ish; we didn't have to wait too long for a mitigation opportunity to appear in the original.
Apocryphal Curse scales directly with our power. Destructor does too. At that rate you will just be running headlong into getting fucked over by your own curses. You can't just powerpowerpower your way through, you need to actively work at mitigating the Curses.
Let's recap Gisena and Curses:
Decimator: Gisena directly reduces power of the Curse - remember that this one needs to be actively curtailed lest we just lose. Gisena allows us to ignore that and leave it for her advancement to keep the Curse in check. Additionally, in this particular situation, we can let Gisena act as our governor while we are in other Realms, thus focusing Decimator on things other than the kingdom we are supposed to rule
Tyrant: Gisana directly reduces the power of this Curse; much more importantly, however, is that she can serve as our agent when talking with other people. She has excellent Social, so we won't feel bad to let her do it, and that would let us ignore many unpleasant situations
Indenture: If you have companions you must mitigate Indenture so you can bring them along; with Gisena this comes prepackaged, so you don't need to spend time and resources finding a way to mitigate the Curse. Her own charisma might help us with finding additional allies too
Apocryphal: As with any other, Gisena reduces this one too. In particular, she has amazing synergy with Accretion/Blade as both reduce Apocryphal and Blade also boosts Gisena greatly
More than anything, however, Gisena makes game much less annoying to play. No having to choose Curse Mitigation over cool powers because we did that in char creation, no having to deal with Tyrant bullshit(as much) and no having to shit into wooden putty instead of state of art magitech toilet.