I got the impression, warranted or no, that the entity responsible for the isekai was also potentially (ben/mal)evolently rewriting the book upon the addition of a new transplant so that even half-remembered fragments would turn out wrong.

(Also, possibly, that Rae liked the stage version a lot and it was very far away from the original text.)

I think you're right that the stock characters of the story, before encountering the transplants, are meant to be wooden and tropey, and that still comes through. Maybe the full finished novel will be the one written when all the roles are filled and everyone gets one last run through the narrative to save themselves.