Alicornutopia

I forget whether or not we mentioned her having a mental privacy thing. In the original plan she would have faced the Simurgh more and developed that, in which case the answer was going to be that her background level of immunity is lower than when she's actively defending. And Eden > Simurgh, of course. It would have made a lot of sense for her to be actively defending when Eden woke up, given Cauldron's admittedly unreliable warnings, but remember that Scion was using a "that's not important" power at the time. (He's also his precognition, checking for a world where Kithabel is convinced to play along, hence him acting a bit out of character with the talking and the showing her Cauldron's dungeon and the using that power in the first place.)

They aren't going to Kithabel's world because they can't get there under their own power (not an Earth/outside their multiverse). Once Kithabel can get them there, they don't have any reason not to go. Useful more for sorcery (gotta catch 'em all) than for the universe itself, of course.

Note that this does mean they've solved the cycle. The genocides can eventually stop, species-wide, as soon as they get enough sorcery and someone convinces them it's a good idea. Technically a happier ending than Worm.