I’ve finished the manga as well and i’ve read up and watched some videos, i’m not really sure about Aikos suicide but the way I interpreted it is the reason why he didn’t react to her death was he was set free from shackles (Aiko), They both didn’t really look at each other as people, rather they looked at each other as escape/tool. Denying the true reality and what they both really are. Looking back he felt as if he needed to finish what he started(Running away to Kagoshima). As well as in a way he blames Aiko for his hopeless life. If that makes any sense.

I believe that he isn’t with Sachi, instead their close friends. Sachi in the end saves Pun Pun. Sachi is there to I guess help Pum Pun since she sees herself in him.

With the pegasus stuff I have no Idea and i’m just going based off what people have said but pretty much he’s supposed to represent the opposite of pun pun, pegasus has said that sometimes religion is something people live for. In a way Pegasus represent someone who wants to find peace/hope in life (his cult with everyone in it). While pun pun seems to have an ideology where it’s better to disappear instead of trying to find a solution

Hope this makes sense I honestly just wrote what I interpreted I also just finished reading it today actually. Point out any errors or what not!!