Okay, this chapter is to answer some of your questions.
First,
Why the MC adapted so quickly to his new situation, not taking few weeks? At least this will be more reasonable.
Answer: Because from the beginning, the MC was affected by Kafka's personality unconsciously and that became more prominent when he received her memoirs.
But of course there is a reason why he was affected by her personality, but that will be revealed in the future, I could only give you a hint.
It's related to their soul.
That's what I could say for now.
Second,
Is there a reason why the MC became a woman?
Answer: It's related to the hint I gave in the first question.
If there any more questions, please ask otherwise, don't bother.