The datamined Withers text |Thou wert called here. Some from above, some below.| definitely speaks of some being called from either the Hells (below) or the afterlife (quite unlikely but not impossible). It would apply to Karlach alone, or with Tav and/or Wyll.

My only concern, and I have it as I'm used to cop-outs in games and tv media especially, is that they'd summon Karlach for this briefly and not as a means of saying they truly got her engine set and now settled in Faer�n. Since she was able to get it under control for a little while during the story, she'd maybe be able to periodically return to the upper plane, Will Turner style, before eventually having to get back to Avernus to stabilise the wretched thing.

I hope I'm wrong tho and my anxious ass will be proven wrong. I'd too prefer a better story/questline solution ingame than having them go to Avernus and work on it offscreen, but as long as she lives and gets to have a happy ending sometime later, I'll be okay.