This scene actually parallels one form chapter 25, where Honami walks to Konatsu’s place and we can see the Konatsu is looking out the window with the curtain shut.
Oh darn, I hadn't noticed. Posting for everyone to see:
https://dynasty-scans.com/chapters/nettaigyo_wa_yuki_ni_kogareru_ch25#4
https://dynasty-scans.com/chapters/nettaigyo_wa_yuki_ni_kogareru_ch25#5
https://dynasty-scans.com/chapters/nettaigyo_wa_yuki_ni_kogareru_ch25#6
Konatsu was at the curtain but it was shut and it was raining on top of it, so the "beautiful moon" couldn't be seen in two ways, plus Koyuki went there without communicating anything first and writing a text that didn't invite discussion afterwards as opposed to how she would call her first in the latest chapter. The line "I'm such a terrible senpai..." itself was also a callback to a previous chapter when she thought she was imposing herself on Konatsu. If you really look into it there's a crazy amount of stuff like this.
I'm not sure we'll get more about Koyuki's and Konatsu's past, I think it will depend on how relevant showing specific episodes from it would be to the current situation. For instance, I don't think it's necessary to explicitly show what happened to Konatsu's mother, it can be pretty much surmised from what little we've seen and it's clear how it affected both her and her father. As for Koyuki, I don't think there was any single noteworthy event that made her into who she is, it sounds like she gradually distanced herself from others due to the pressure of being a model student (as opposed to, say, the reason why Touko in YagaKimi became one). Of course I would love to see how she came to love aquatic life and take over the aquarium club.
The frog starving itself to death is an analogy for it starving for something more in their relationship” if Koyuki is the frog and Konatsu is the salamander, then maybe she will get rejected (initially) and we will enter an arc where the emotional love turns into one that is non-platonic.
Although the series flipped the perspective of who the salamander and the frog are in the analogy with the short story when Konatsu tried to hold Koyuki back from going out with her classmates, I'd still say the roles align more to Koyuki being the salamander and Konatsu being the frog. When they finally had their heart to heart in chapter 26, Konatsu thought that she always wanted to be a frog so she could be with the salamander, that she thought that was all that connected them, but then implies there will be more to their connection from now on. The salamander is shown swimming away together with the frog at this point. I took this image and this entire scene to mean that the analogy with the short story has now been resolved with the animals finally opening up to each other, the salamander managing to leave its cave thanks to the frog and them setting off together to unknown waters.
That's why I think Konatsu wouldn't reject Koyuki's feelings at this point if she were to confess. Her dependance on Koyuki is no longer one-sided, they both expressed how they need one another. To this end, there's been a lot of set-up about the time when Koyuki will have to leave for Tokyo to start university. In the chapters where Konatsu's dad returned, she came to the realization that even though she'll feel lonely without his physical presence, distance will not break their bond and she can face her loneliness with that knowledge. Koyuki's father tells the same thing to Konatsu's in regards to their daughters' relationship once Koyuki has to leave. And Konatsu herself wrote "frog" as her career choice in this chapter. I wouldn't take that as a simple joke, she's saying she wants to be by Koyuki's side even after graduating. Maybe this could be what they'll talk about next chapter since Koyuki seemingly wants to have a proper talk about her leaving for Tokyo after graduation, and Konatsu "won't be deterred" from expressing her intention of catching up to her once she also graduates.