Your thoughts here mirror my own. Although I do recommend this alongside よつばと！ and 三ツ星カラーズ, the Kansai dialect is pretty much the one sore point for absolute beginners who don’t have the benefit of guidance on translating to Tokyo dialect. For those with such guidance, it think it makes for a great introduction to “fiction Kansai dialect”, the kind they’ll most likely encounter in manga and anime. (I don’t know about in live action fiction.)
One reason I took so long to reply to your prior question (aside from accidentally closing the tab I had open as a reminder) was because I knew I’d couldn’t give an immediate answer off the top of my head. I had to re-read the panels leading up to it, and look into what I could find on it, to give my interpretation.
That’s happened more than once with this series where I was unsure of some grammar. But I’d still recommend it to early learners, because (as I’m sure us readers all know), we have to be able to tolerate ambiguity on this journey.
Has it really been that long? Wow. (But imagine waiting for new volumes to come out in the store to buy!)
Regarding the speed of events at the ending, I will admit, when I first read the series, I wasn’t certain if there was actually a path to saving her brother. I think my favorite scene in the series is when Kanami’s brother goes to use the phone, which is right around the start of the ending sequence, so that likely does cloud my judgement on the ending a bit.
I may have missed something, but I’m under the impression she’s their biological daughter.
Regarding the blackmail, that seemed like the weak point of the story to me because I wasn’t certain the why behind it. I may have missed something vital, though! (This was my second read-through, but perhaps I tolerated ambiguity a bit more than I should have at some parts. I appreciated people questioning parts for this reason!)
Edit: Maybe if Kazutaka had issues with their parents, and didn’t have any friends, and was protective of his sister, it’d made sense why he’d have the one good person in his life as his smartphone background. The classmates could have threatened to make up a lie about it, although the whole “let me use you as a punching bag to let off my own stress” thing did feel a bit contrived, if I understood it correctly.