One major flaw of this story is the relationship between the MC and his fiance which is stagnating. I know that many people hate her because of abandoning the MC at the beginning. I actually think that this was the intention of the original mob Butao. Looking at his relationship with his family and how he behaved at school it feels like he was self-sacrificing so that his love could live a happy life (maybe he thought he was no good because of his skill). In the talk with his family we learned that he entered the school to protect one person which is probably his fiance. The MC and Butao seem to share their feelings somehow, so he is not interested in other girls. Deep inside he only loves her. But still he's so focused on maintaining the original story that he's willing to get NTRed by the game MC thus ignoring the feeling of Butao.
The story between MC isn't progressing (in my opinion it was the driving force for original mob's actions) for an eternity. He has feelings for her bit doesn't act, she gets glimpses of his change but keeps sticking to the game MC, but we have the usual lack of communication (it really feels like rental girlfriend; one step forward, two steps backward). They just lead separate lives (which can best be seen when they walk to school). The few occasions when they talk don't progress their story but are just depressing at this space.
The real problem with Kaoru is that she's a main heroine who doesn't support the main character she has the spectator role and looking at the situation from the NPC perspective maybe if Risa was the main heroine the readers would not bother with her.
The whole MC and Butao being separated entities are not set on stone sometimes he feels like he's merging with Butao, he already assimilated all of OG Butao behaviors to the point that his family doesn't even notice something is off about him.
I actually think that this was the intention of the original mob Butao.
Original Butao was an asshole.
He has a heart of gold when it comes to protecting his family, but that's it, he could not control his feelings and was extremely jealous and toxic in the middle school he even slandered her and in the game he also spread bad rumors about her and made dirty jokes like she was Yuuma's slut with the added bonus of sexual harassment.
The marriage contract was a device to keep her interacting with the MC despite all the crap he did towards her.
There's no self-sacrificing. (his actions towards her contradicts it like forcing her to wake him up and go to school together).
MC is aware of all the mess Butao did towards her in middle school, so it's kinda hard for him to change that perspective when she acts annoyed towards him, specially when the MC is aware that all the times she approaches him to talk she has second intentions.
The problem is that - even though it's understandable to some extent that he acts like does - if the story continues like this it's just a boring read. He lived a pathetic life in his old world and after being isekaied he ... still lives a pathetic life.
Don't get me wrong, the series has a lot of potential and it's a good thing that he doesn't replace the game MC immediately (even though he's already stronger than he and his entire party) but (in my opinion) the development is just too slow (especially his relationship with his fiance).
If he uses his OP skills and knowledge just to nullify the other players tries to change the story so that his character can live a lonely life in misery just because he's an NPC it becomes boring and depressing at some point.
He doesn't lead a pathetic life, there's plenty of material with him lunching together with Risa and Satsuki, sometimes he does lunch with Kuga and often goes into dungeon dives with Tenma/Kuga.
Currently, he has three party, his family, EEE (Risa, Satsuki and his sister), and Tenma's.
About development, Kaoru is going to find out the truth by herself and confront him probably after the author decides to "merge MC with butao" or go with "separate entities".