1. The story is about a lot. It is about the entanglements of the people of this 'suicide' town, all of them are dealing with depression and emptiness, in other words, they are all struggling to find meaning to their lives. Reiji's struggles are the main focus and an interesting point of view when looked at in comparison to the other characters, especially the adults imo.
Some of them, like Nagi, believe that their lives are just an empty abyss, that they can never find meaning ever; that's why she was so ready to die with Reiji, she really means to kill herself. She has given up her struggle and just wants to disappear. While others, like Chako, actually have goals that they want to accomplish. She is struggling with her ambition and her overall view of herself, all while trying to do what she thinks is best for her.
Reiji is an interesting MC because he is floating in the middle of those two 'extremes', on one hand, he wants to (or you could see it as 'he is forced to) help out his family and especially his mother (who was more likely abused by her husband in the past (btw I'm wondering where he at now? Did she kill him? Possibly...)) with taking care of his grandmother and his shut-in brother (who has the energy to be an asshole and no energy to help his mother out, smh); on the other, he wants to be free of these ties that are holding him back from living his life. He is also struggling with dealing with all these different directions he could go, he could stay and work for his 'friend' Gen's family (which his mother and Gen are pushing him to do), he could go to university in Tokyo and leave everything behind (which Chako and his teacher (what was her name...? Oh yeah, I remember, its Pedo-sensei. smh) are pushing him to do), or he could just let it all go and die (which Nagi wants him to do with her).
This story's main focus, though, is how Reiji deals with all of this, and how Reiji plans to come to terms with what he wants.
(((Imo, all of these people are wanting Reiji to do what they want, his mom wants a break from the family, Gen has his weird attachment to him, Nagi wants someone to die with so she chose him since he seems similar to her (they knew each other for two days and has sex once, thats definitely not enough to justify her decision to include him in her suicide plan, smh), and the teacher wants to satisfy her savior complex (and her sexual desires too apparently, my lord whats wrong with her). The ONLY one who seems to care about Reiji and has his best interests in mind is Chako, and she is best girl. Literally fuck everyone else but her and Reiji, I hate all of these other characters.)))
2. Minenami Ryo is known for deep seinen and more mature content. Their works are pretty good at this kind of story telling, if you read 'Himegoto' its pretty similar (though this story is muuuch better imo) oh and don't read 'Hatsukoi Zombie' unless you want to be disappointed, smh)).
3. I do not believe this is that similar to 'No Longer Human', the characters and main struggles of Reiji and Oba are too different. Oba tries to mask his feeling of alienation by being a fool and fucking around (literally), while Reiji is a quiet boy who keeps his head down. Maybe once this manga is fully completed, there can be a better comparison between these two and how their stories unfold. For now, of course there is some similarity, but not too much.
However, that is with the main characters, other characters are better comparisons to Oba and the one best suited for a comparison would be Esemori. They both are artists, they both hide their real selves from everyone else by constantly fucking around (Reiji's mom even called Esemori a 'bad man', and I believe it too, bro really almost fucked a 17 yo girl, smh (actually ALL the adults in this series are fucked in the head oml)), and they both tried to attempt lover's suicide and it ended up in failure, leaving them both guilty and more empty than before the attempt.
Like I said before, once this story completes, there could be more similarities between this and 'NLH', but for now this is all I can say.