So the baby/children theory was right...

And Queerats really did find ways to control fiends, their best human annihilation weapon.

Tomiko hinted there's a solution to killing fiends, but with luck. My bet is that it takes someone with her ability to manipulate their own DNA into erasing the deathfeedback... but only if the brainwashing part can also be removed.
I guess Tomiko can manage the DNA part, but never worked towards the BW part.
Saki might be the only one who does master both, although we never really witnessed her working on DNA manipulation.