The problem for me is that the story as it's been presented so far makes no sense, unless we get flashbacks to missing scenes or internal thought bubbles. In which case I would still say the writing sucks ass because that's retcon level writing, the internal logic of the stories and characters should be consistent throughout. Not only after a later point in the story.
The basement from AoT would be a example of this done well, it doesn't rewrite what happened earlier in the story it only makes clearer what was actually happening and all motivations for the characters still make sense.
Also I didn't read either of your spoiler tags because I don't wanna be spoiled I don't think future spoilers can change what I have a problem with as I've been reading so far.
The MC doesn't fail, it's just that it's a bit similar to Leon with Mobuseka. In the Mobuseka Leon wants to follow up the story, but things always go sour and when the events are almost certain to go to a bad ending he decides to interfere and save the day.
Here MC(Souta) actions area always justified later, here we can see he was right to not standing out because Tsukishima is sabotaging class E for reasons unknown (he interfered with Kariya and Suou) and he would be targeted if he decided to help them.
So what happens is that as opposed to Mobuseka, the heroines who are close to Souta who ends up saving the day. Here Imouto decided to help class E without her brother's permission and that attitude saved his classmates. (who knows what would have happened to the main heroines Kaoru and Sakurako, it's obvious they would get raped by that fatso).
So in short, Leon from Mobuseka resolves things directly while Souta solves the problems indirectly.
Here, Class E is going to lose but Souta manages to defeat that Giant Devil, but he gives it's magic stone to Tenma, he does this to not stand out, and also he thinks that losing the competition makes no difference to the story.
However, Tenma delivers the magic stone to the teachers and tell them it belongs to class E. Giving them enough points to win over Class D.
Because of this victory, it ends up lifting everyone's spirits, including Kaoru's who was looking sad and hopeless.
From here on, she changes her opinion of him definitively and acknowledges he's strong, but also she finds out he's hiding several things from her and she's now determined to find out the truth about him.
Also, in the novel, she no longer wants to cancel her marriage contract with him and instead feels a bit lonely without him (he's been constantly avoiding her despite several invitations to dive in the dungeon and spar together.)
No, if you read in the beginning he's quite happy with that lifestyle. Until Zola wants to marry him off to an ugly noblewoman to have him killed off and get pension money.
He goes on an adventure and obtain Luxion, and still he doesn't want to change the plot nor interact with the main characters.
He decides to save Angelica because Marie rushed the story.
But even then he goes back to being passive, and he only manages to act when the Kingdom is about to get destroyed by the Principality.
The same trend happens in the Alzer's arc.
When he decides to act when Noelle is about to have a bad ending (she gets kidnapped by Loic and is about to be forced to marry him).
Only in the final part, he learns his lessons and starts doing things on his own.
The writing is just kind of ass, Isekai authors love exploring enforced hierarchy's that MC is always functionally outside or at the top of but doesn't like doing the leg work to justify MC having to struggle in a meaningful way, have self confidence issues in a realistic way or justify why he wouldn't utilise his knowledge and power.
That's the problem when the author justifies MC's actions later...
In the Novel after the competition arc. Kaoru is desperate to be strong and wants to reconcile with him.Souta still wants to stay away from her for no reason other than (can't change the plot, but we know the plot already changed).
However, it's justified because soon after Tsukishima chimps out and challenges one of the eight dragons to a duel and also decides to take control of the school and build a harem with heroines.
So Tuskishima starts being more aggressive on his flirting with Kaoru in an attempt to win her over.
Kaoru practicely being a gold digger is a known trait of hers established by the author (probably unwillingly) since the start.
Her attention immediately switches to the most powerful man in the room. First it was the protag (of the game) then Tsukishima then the MC (because strong people start to flock to his side) then the masked man who beat Tsukishima (MC in disguise). She is weirdly primed for being THE NTR bait in the story and also the reason majority dislikes her. Manga retcons are most likely the attempt to change that perception.
He is thinking "if i'm the strongest, why would i give a fuck about bunch of NPCs (not entirely wrong)? I will destroy the hiararcy and establish myself an harem with my power and knowledge". Yeah, he is also a little creepy with his obession with Kaoru.
He also doesn't think any of this is as reality and treats everything as still a game (therefor doesn't care about worlds ending if things goo off script) until after got beaten by the MC.
