Gonna have a go at explaining Jisatsu.
Its an adolescence story, the whole professor thing was never real, it's just his escape from the reality he feels powerless in. Natane appears in there because he envies her ability to live her life as a protagonist (mentioned during their first meeting before Midou appears) - she is in control of her life.
The professor is the representation of him being shackled by the world and it's laws, that he feels powerless to change anything in his life and has to accept it as it is - that he's insane, that life sucks and he has to endure it. Him 'killing' the professor is the realization that he can influence things, all the happy worlds are ultimately a rejection of escapism - sure, he can imagine all of this and ignore reality, but it doesn't feel right, so he instead chooses/has to accept the imperfect, shit world. Except this time not as a mere observer, but the one in control of it - this time he is "creating" his own world. It's also why (in the end) he thinks about having a purpose, moving towards something and that 'he won" just by reaching the girl - because he proved he can change things.
As for the ending and salvation we were thrown religious scraps in the Samomo conversation regarding confessing their sins etc, but im not sure how solid that is and whether it can reliably lead to anything other than the ideas of forgiveness, relying on others and kind of sharing your world with someone else. The usual.
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