You're brushing up against something I've long had in my notes as a half-formed suspicion: There's something off about the way the author either understands or portrays human intimacy. Not exclusively sexual, though that's a little... eh, also. But like, any kind of emotional connectedness in this manga is tainted, misaligned, or has a cloud hanging over it... or is simply absent. Its world should, at this rate, be a lonely one for Riku, because we never really see him confide in anyone about all the shit people are putting him through. And not gonna lie, he comes across as a bit of a psycho for not being affected by it. It's hard to put my finger on, because we're used to just assuming that things exist normally unless otherwise specified, in fiction, and the nature of this story and the way it's structured is that if anyone ever talked about the central mysteries of the series, they'd evaporate.
Not that it needs to be that way, mind you. This isn't a very (intentionally) psychological story at all, so following normal rules, allowing dialog with other parties is super useful in a lot of ways, and depicts a more realistic life. You would assume that if that's not being done here, there's a good narrative reason. And I think you'd be wrong in that assumption, is my point with this. Contrast with something like Mysterious Girlfriend X where undermining intimacy to create an unsettling atmosphere of alienation is deliberate and effective?
You mention the manga being dog-water awful, and I agree, overall, but it's not ONLY awful. Most of the important 'mechanical' writing skills you'd expect of a professional are like, really quite competent to above-average. Good, even. And yet there are really notable exceptions. When you zoom out even a little, the story that those skills are used to tell is very, very stupid. It sucks that I can't think of a more fitting way to describe it, because that's typically used so casually, but I mean it from the bottom of my heart: It's fucking stupid. It's not poorly done in a way that's consistent. It's surprisingly bad. It goes out of its way to do dumb things that it seems really proud of. And that does make it a certain kind of interesting to me, because it's like a fun little extra mystery that the author included completely unintentionally! What completely basic thing will slip through this week? Why didn't anyone in the process say "no, that's dumb?" All these questions and more.
Like, as an example, allow me to point to chapter 33 where Umimi finally snaps and demands answers from that piece of garbage she's trying to hook up with, and it's really jarring because she's calling out the absurdity of the lack of progress which we're just taking for granted at this point. We're numb to how terrible he is. But she finally has this big emotional cathartic moment where she unloads years of pent up feelings and forces Riku to take her seriously and look at her. Finally, someone is making sense and trying to get answers, and yet... And yet. She has no pants on. And has deliberately just removed them. For this purpose. Her moment of character growth is spent Porky Piggin' it, lips to the wind, cheeks in the breeze. I think the author intended for this to convey her desperation at having tried all the things that made sense to her. I think the author intended for this to be hot. I don't think the author intended for what could/should have been a relationship-defining moment of emotional connection between these two characters to be that absurd and "wait, what?"