Putting aside the fact that Yuigahama doesn't get any backstory or development because she's only there to make the other two more interesting,
How did you get that idea? She doesn't get any direct developement, but more a sort of developement in understanding of the other two characters and their dynamic. Does that not count as developement in your eyes?
Also why do you feel like her only real reason to exist is to make the other two more interesting? She is her own character, he own flaws, her own strengths, her own quirks. To me, she first seems like the typical archtype but with so much depth that she becomes a great character.
Her way of thinking and her actions give a nice contrast to the other two main character, similiar to Hayato.
Yukino at least has experienced being shunned for an understandable reason.
Do you really think that Hachiman's reason is not understandable? This sort of self protection mechanism definitely exists among teenagers, even for lesser reason. There can be ridiculously small things that can lead them to a smiliar behaviour. You don't need to be abused or under constant pressure to hide yourself in a shell you create.
Hachiman who only seems to be against the world to be against it.
The whole point of the show is to portray how stupid Hachiman's behaviour actually is, but also to show how human it is.
I have a feeling it's significantly easier to understand the characters if you are introverted yourself, which I get the impression you are not (please don't take any offense in this I have no idea how else to express this). It seems like you have problems understanding the characters, how their introvertedness(?) affects them and how hard it can be for people to overcome that and act socially. It's just so much easier to tell yourself you don't need anything like that, that the others are wrong etc. (Source for this is myself to some degree but I have also had some very intense conversations with people with similiar, but significantly worse problems.)
OreGairu shows you very clearly that if he wants to have a conversation, he can easily have one, but he forces the abstract and negative vibe onto everyone and every subject.
Exactly. He can only talk because he does that. That makes it so much easier for him. It puts a distance between him and his conversation partner, and this distance allows him to talk, but not really have a conversation... it's hard to express.
being able to have a normal conversation is not something you learn by being alone for a long time, it's either something you have or something you learn and grow into. That means that there has to have been a reason for him to grow apathetic of others, most likely being rejected by the girls who never texted him back.
He is not apathetic. He tries to create the delusion that he is apathetic towards others. He wants to have friends (see episode one where he immediatly asks Yukinoshita to be his friend). He wants to have someone understand him, he wants to be able to share his pain.
There were several parts where he was deeply affected by what he did. For instance the last episode of what he did to get the chairman to hold the speech. There were no words, no monologue (if I remember correctly), no action. The complete lack of all of this speaks for itself.
As you have probably noticed already, I disagree that Oregairu was "built around the wrong loner". I feel like from his perspectives it opens so much more possibilties, because of his very limited, and very biased view. Without him as the MC we wouldn't have seen the depth behind Yuigahama, we wouldn't have slowly understood Yukinoshita and her struggle, and Yukinoshita's sister's character.
About the plot points: They didn't bother me at all. In fact, I thought it was a great choice, better than anything else they could have chosen. All of them were portrayed through hachiman's perspective, meaning all of them were used for furhter characterisation and understanding of his character. All of them were kept interesting by the characters. Pretty much all scenes had the familiar setting but all went different than normally because of the very different character's compared to the usual romcom.
Hachiman did grow. Not far, but that's understandable. Someone with such a worldview doesn't become completely social in several months. Breaking free from a delusion you have set for yourself is hard, even more so when that delusion the way you want to see the whole world around you.
It's like there is a small crack in his facade, and now he is able to share something of himself, something he wasn't able to do before. SOmething he never imagined he would be able to do. He has developed significantly more than he thinks he has. He still thinks he is the same, or at least on the surface he does, but he is not. He still clings to his delusion without noticing that he is already with one step in reality.
So in the next season I assume we will see him come to terms with this.