What a breath of fresh air in the genre of unrequited love! Some readers found the other chapter sad, but I think there's little to no sadness or angst in this story. My impression is that the message of the story is that love is not always such a monumental experience and does not always have to be fretted about—or make you depressed, if it is unrequited. Like most everything else, most love will come and go, and in many different shapes and variations!
this is true, however, the scene where Eiko-senpai's hair gets caught on Shina's blouse symbolizes their connection for that brief moment. Where Eiko wishes to stay connected to this one person (Shina), Shina is working to untangle the bond and re-enforce the point that it is only a brief connection because her heart belongs to someone else. What is sad and indicative of the points you are making, is that from the prequel "Love Prep Room," Eiko's feeling toward Shina evolved from nonchalance to longing and possibly jealousy towards Satsuski.
While I too would not tag this as angst, i would tag this as sad because Eiko is trying to gauge Shina's reaction by 1) saying she would cut her hair, 2) by trying to extend the physical connection (i.e. making Okui have a second orgasm), and 3) attempting to embrace and kiss Okui much longer than expected. after each of those instances, go back and look at the expression that is drawn on Eiko's face. Point in case is the last page ( http://dynasty-scans.com/chapters/fever_retreat#16). On the top panel in the dialog box, Eiko's desire is revealed and in the middle and bottom panels , her disappointment at having their connection severed is shown.
but heck, this is just my interpretation. while for the most part I agree with what you have written, I just argue that there is (if only in this story) a sadness element. angst? no. that could come in yet another follow up story.