It's funny how brazenly the LN authors copy even the core idea and setting of another series, as it was nothing peculiar. Maybe the reasoning is that since, for example, high school drama series, of which there must be tens of thousands, can't have that much variance, in the end, then why must fantasy be unique? For example a gal falling for an otaku is a pretty specific core idea for school drama (and much used), so maybe taking over an initially strong support character doomed to die is comparable.
Aside from that rambling, I have to say Elder Scrolls Oblivion had a much better idea/mechanism for closing dimensional gates.