I swear i thought the first person who was with umi-chan was a boy and i thought she is her boyfriend, so i jump into a conclusion that this could be an NTR. From what i just comprehend from reading this, this is the best of the best.
There's a definite case of, "one should never aspire to conduct one's relationships like this for a number of reasons—but these two should be fine," in here.
Like, all of the friends around the edges that are dubious about the whole affair, are quite sane to be extremely doubtful. (Which isn't to say this doesn't end up heartwarming to the point of being silly, mind.)