The manga is about these two characters and their relationship and yet after reading it we still don't know anything about it.
We know what the author wanted to convey, which is that Kei believes her partner is emotionally unfaithful to her, and feels sad and lonely. You say over and over again that all the plot points are ambiguous but that's not the case. The details are ambiguous.
Their pasts, presents and futures are all left in the dark with no clues for us to find to figure out what the hell even happened.
For one, there are clues. We can look at Kei's hatred of tobacco smoke, and imagine she had a traumatic experience with a man who smoked tobacco threatening her relationship. We can look at Sachi going to the store to buy new headphones, and infer that she may be trying at a new start with Kei, unbeknownst to Kei. We can look at the last page of Kei smoking a cigarette and glaring into the pitch blackness, and infer that the relationship is ending or at least not going anywhere happy.
I don't love this manga by any means, but there's definitely an intention, a feeling conveyed. Details periphery to that intention being left to the imagination are NOT a sign of an inept writer.