I mean, winning the competition is also important to Yori. SS lost during the audition and Shiho told her that "she has a long way to go" (I think it was the only time they spoke, lol). Not to mention, she even wrote a new song for the occasion. Whatever the case, Yori is lucky to have the plot armor on her side.
At this point winning the battle is pointless, there's literally 0 stakes, heck even if Sunny spot were to lose as long as shiho listens to aki s song the plot could move as if nothing happened, besides is not like yori has something to prove to shiho as far as I'm concerned the only reason yori wanted to win was to help aki, the only time yori and shiho had something that barely resembles a rivalry was went shiho got a bit touchy Feely with hima to make yori jealous
Honestly, I thought you were being sarcastic at first. Shiho is the person who has suffered the most throughout the story, as she's someone who has always been burdened by expectations, pride, jealousy and guilt. I'm not denying the bad things she's done (I do believe that Shiho will eventually come to terms with why some of her actions were wrong and show accountability for them), but readers are overreacting in their criticism of her and this long Arc. The person she's hurt the most is herself, after all.
I mean you're kinda right about that last part, she's been an asshole and a pain in the ass to every person in her life but is not like she has done something morally reprehensible.
What puzzles me is why the people around her keep putting up with her shit, like sure she's a "realistic teen", but other "realisticas teens" would already be done with her, even if her character makes sense it's weird how everyone just shrugs their shoulders and go "good ol shiho am I rite" whenever she acts annoying.
As for her having a change of heart I highly doubt it, I'm sure that the next chapter will be about aki confessing and apologizing to shiho and shiho will look her over her shoulder and go "hmp I guess If you're that desperate I can forgive you and give you a chance to date me" and then every shiho fan will just write easays justifing and rationalizing her actions and about how aki and SS were actually the bad guys all along and shiho not only deserves the apology but also doesn't need to apologize