I definitely agree that there was poor communication (a staple in nearly every romance anime) between Ryuji, Taiga, and Minori during the entire school year regarding the subject. However, I gave points to Minori because she did confront them a few times and tried to get to the bottom of things. Each instance, however, Taiga and Ryuji responded by adamantly denying there was anything between them. I find it remarkable, that Taiga not once told Minori how he felt. You'd figure that girls who are best-friends would let the other know about something like that and not be so coy about it.
At the end of episode 2, it seemed like Minori was possibly going to confess to Ryuji after she apologized to him for the [first] "misunderstanding" on the rooftop. But because he was preoccupied with Taiga, he walked away and didn't give Minori the opportunity to say what she wanted. I believe after that, she remained skeptical (especially following events such as hearing Taiga scream, "Ryuji is mine!" upon saving him in the pool), which likely influenced her decision not to confess and ultimately leave it up to Ryuji.
Minori's depression was the result of her spacing out during the softball game when thinking of Ryuji and the mixed up signals she continued to get from he and Taiga. This is when I felt the most sympathy for Minori and why, in part, I think Taiga could have done more to consider Minori's feelings by connecting the dots and backing away from Ryuji a bit to avoid any more "misunderstandings." At the beginning of episode 21, Taiga even admits to Ryuji that she had been acting like an idiot for hanging around him all the time while trying to get him and Minori together. This, according to Taiga, explained Minori's numerous misunderstandings as well as her recent avoidance behavior and rejection to Ryuji. At this point, Taiga finally realizes her actions and behavior were inadvertently sabotaging their match-making efforts. She then tells Ryuji she will no longer visit his place and instructs him to chase after Minori to get her "real answer."
I wouldn't say Taiga "back-stabbed" her best-friend, Minori. However, I will put it this way: Taiga should have recognized a certain line that she ought not cross so that "misunderstandings" between her and her best-friend, Minori, did not evolve. However, because she wasn't careful enough, these misunderstandings escalated into rumors and led to Minori having emotional issues such as depression and avoidance behavior. By this time, Taigi should have been perceptive enough to connect the dots, and realize what effect her actions were having on her best-friend. Thus, Taiga's response should have been to distance herself from Ryuji and give up any idea of being with him for the sake of Minori's friendship and well-being. That to me, would have been the mature decision on Taiga's part. Of course, Ryuji should have been just as perceptive; but as is the case, he was playing the role of the oblivious, scared nice guy perfectly.