This latest chapter puts a bunch of things in context.
This is my interpretation, so take anything I say with a grain of salt.
The relationship between the witch and the priest mirrors Marisa and Reimu's relationship. The priest (Reimu) knows that they shouldn't be friends with the witch. It goes against their code where they would exterminate those who break their religious beliefs. Despite those beliefs, they maintain their friendship.
When Marisa loses her powers, Reimu is at first relieved but slowly loses interest because Marisa is no longer the person that she became friends with. That curse, first established by the priest, awakens Reimu.
Because the priest and the witch, after those books were published and sent to her, brought context to the title of this doujin series.
The priest is the magician, the one who made the curse, and the witch is the fake, the one not responsible for the curse. The priest (Magician) who loved a Witch (Fake).
Bringing this post back to the present, Reimu has become "corrupted" by the priest's books/curse and has become the magician because she loved her relationship with Marisa the fake human witch. Reimu was never corrupted by the book, it simply enhances her previous emotions because she misses her old Marisa. Even if most of the village sees Marisa as Kirisame's daughter, Reimu only sees Marisa as the human witch. So when Marisa "learned" the abandon food skill, Reimu's life was already altered and after Marisa lost her powers is when the camel's back broke and allowed the curse to possess her.
While this story will most likely revert to the status quo of 2hu lore, it was a great read and I look forward to the author's next work. I'm very curious about which character they are going to focus on next.
