Hi maybe fits here:
So after Adachi bleeded her feelings to Shimamura just replied Adachi is annoying and then they meat up to at the pool where they kinda come to terms.
I'd like to answer that more broadly, but it would turn out to be me rummaging instead, so i'll make that brief.
1) Q: Shima haven't developed any feelings towards Adachi.
A: At the time of v6 yes, she haven't developed love. But she still regarded Adachi as a "closest person". Adachi s feelings are more akin to passion, burst of fire, while Shima's are , well "exalted friendship" which is not that stormy, but grows over time , as sprout. Shima "accepted" Adachi's love at that time. About her love, well , it came to existence in v8 in my opinion.
2) Q: Adachi through the eyes of Shima.
A: One of the key points of the series is "play with perspective". Volume 5 played almost entirely through the eyes of Adachi , while v6 is Shima's volume. Though I can't deny that Shima somehow plays superior to Adachi. In v5 its amplified because of how we influenced with Adachi's perspective and therefore looks dreadfully, while in v6 Shima looks like doesn't care about it as much as Adachi.
3) Q: Adachi's courage.
A: Its simple. For 5 volumes all their relations lived on her, because if Adachi quited, or lost at least once it would all be over. Because yeah, its her in love, not Shima. Shima is the person who would not do anything for anyone if it requires her to go out of comfort zone. ( At least WAS. Now its not one-sided any more). And considering Adachi's passion, and that for her it is something that can be once in a life, she absolutely has no rights for any suicidal moves that would end it. And so, sometimes she can't do anything against Shimamura. Also I don't think a "dog" was ever meant as an insult here.
Its my personal opinion, not a "truth" of any sorts. At least it is "the-things-i-prefer-to-believe". There is also "cynical-meta" version if you interested.