To be fair, she was probably managing to keep mother in law and grandma at bay over the last 2 years and several months because nothing happened to ML. She was probably being treated about how we see at the beginning of the story; fulfilling her role meant nothing more was ever directed at her than harsh schedules and gazes. Which, while better than what she WAS dealing with at the hermitage, is also still not the environment for proper healing from the trauma she has.
It was only when ML got into accidents that the facade they'd been comfortably hiding behind to keep the status quo is broken, and now mil and grandma are pretty much out for her throat if she doesn't do everything carefully. Of course her anxiety would severely spike, and make her start to doubt her own beliefs especially with her grandma's mysterious calls (because ambiguity is volatile for someone who is already anxious), and then ML coincidentally being hurt at the same time. That would fuck anyone with untreated anxiety up, and I am genuinely not surprised she internalized all the blame mil has put on her as a result. Anxiety isnt exactly known for being logical, and at this point MC is being controlled by it. So for her, even if she doesn't fundamentally believe in the things she's doing, her anxiety has rationalized that /if/ there is even the slimmest chance they are real, the actions her abusers are telling her to take are now a "better safe than sorry" method ESPECIALLY when her abusers have shown they have no issues invading her autonomy to get their way and she needs to have some kind of self preservation in a situation she feels she cant escape.
TLDR: MC is extremely traumatized and anxious with no healing done; She's not surprised by her abuser's actions, she's surprised about the fact that their controlling methods are working and her anxiety is keeping her from being more rational and she's unfortunately falling into their pace to keep things from becoming worse.
So ML is great and his mom is awful. That is undeniable.
Why is MC acting surprised by her situation? She literally begged ML to be taken as his talisman to come out of her situation. She shouldn't be enduring what she is going through, but she literally sold herself and her body so that she could save her mom. So why is she now acting as if the treatment she is receiving from the ML mom and the shaman is unexpected? She wasn't taken there for her feelings, she accepted the deal because she had something to gain from it.
I am not justifying the ML's mom behavior, but I am saying that she never saw her as anything more than a mean to save her son, and MC knew about that from the very beginning.
Plus why delete the tattoo and not follow the shaman's orders if you are going to be doubting yourself and your actions every second? So NOW she think that sex might be the solution? After 3 years of practically 'ot following any of the directions that were given to them? I am confused? Now you suddenly won't take of the tattoo without permission? Even though you only have two sessions left to complete the removal?