Yeah, this is pretty messed up on many levels.
1. MC's a "child", no she's supposed to be an adult with extensive high level education in a logic driven field.
2. The false saintess is a misguided child, no she herself chose to steal power from at least two (probably more) and intentionally condemned hundreds to a terrible death while smiling and receiving praises for the act. All while being fully aware of that reality. She also admits this to the MC as well.
3. False saintess attempts to kill MC in her sleep, they fight, with MC using moves that could easily cripple/kill the false saintess (glass window exit), MC declares to others around her that she just survived an assassination attempt (Likely ensuring she will be executed), then somehow instantly acts as if the murderously twisted girl was actually a good girl all along.
4. False saintess explains her power is based on "bartering" (e.g. She gives you something, she takes something), then MC promptly agrees to "take" something (magic stone) from the false saintess.
5. How in any possible version of fiction would a single magic stone have the power to send someone to another realm?! (There's no way MCs still on the planet)
MC's earth "history" is a joke at this point and only makes the story worse for it. Feels like the author only had the story figured out for the first couple chapters and is just winging it now.