No, not invincible - just at level 10.
I'm pretty sure she's more like level 50, given what happened when I went Unbreakable against her for three rounds before dying, having not managed to land a single scratch on her 999 health, nor more recently when I fought alongside her for a few rounds against something even worse, and could barely manage to land a hit against the summons being spammed out incidentally by said something.
And yeah, destroying reality tends to trump turning a planet into a sexfest.
Arguable. The Order of the Stick has recently featured some discussions of whether letting an evil tyrant oppress the planet for a few centuries is better than unmaking it and starting over. For the purposes of their storyline, that possibility is rejected, but I can definitely see it being applicable here. The existence of Tira definitely seems to support the idea of mercy-killing as an alternative to fates worse than death, and I would call enslaving everyone as a soulless rapemonster one of those pretty definitely.