I understand that you’re trying to rely on the canon, but that’s exactly where your logic falls apart. Kawahara did create plenty of male characters, but none of them fit the role Inoda serves. Klein and Agil are Kirito’s friends, and using them in such a scenario would completely destroy their established personalities.
Heathcliff is an archetypal "divine antagonist", and any involvement from him shifts the narrative focus from personal drama to a metaphysical conflict. Sugo is a completed and already overloaded antagonist; bringing him back for yet another cycle of exploitation would feel repetitive and creatively outdated.
That’s why an original character isn’t a mistake but a structural necessity. Inoda exists outside the constraints of the canon, which allows the author to develop the storyline without dismantling established character frameworks. He serves as a narrative ‘foreign element’ introduced specifically to trigger a conflict that cannot occur within the existing system of characters.
From this perspective, Inoda’s appearance isn’t a violation of logic but, on the contrary, its preservation: the story gains room for experimental deviation.