Soo....let me get this straight...this guy is some diabolical genius who's in charge of an entire religion, and is also responsible for "entrapping" talented individuals in such key positions...didn't it ever occur to him that a young girl, who traveled who knows how long, and risked touching you and using her gift just for the sake of her brother...will just refuse to use her gift and just sit and watch the world burn to spite you and your religion that murdered him? Heck even if we assume she loved her parents enough to not end the world for their sake, she can just stop casting the spell after 20-50 years, after they have most likely already expired.
Also..why in the nine hells send her only viable replacement(the blindfolded time controlling girl) to her death, prior to making certain that the girl you want to replace her with, is willing to take up on that duty?
It's like throwing away a perfectly good and in working order machine, just because you've ordered a new untested one, from a seller you don't know anything about...sure it might end up working as intended, or it might be a temu knockoff.