Yeah, probably this backstory should've been narrated earlier since pretty much all of us hate Hiroko and have made up our mind on the "she only rejected Ayaka to protect her public persona" plot line
I mean, that’s kinda the point. The author wants you to draw conclusions about the character and get comfortable with that assessment. That way, when the curtain is finally pulled back, you stop to reassess everything that led you to your previous assumptions and make sense of it in light of the new revelation and effectively un-make up our minds and engage with the story in order to see things in this new light. I personally think the author does a great job of it here, especially the way they manage to roll the lingering effects of old social norms (a thing that a lot of readers probably aren’t going to think about since it’s a work of fiction) to make you really realize how Hiroko’s seeming density and cowardice is a sort-of trauma response.
Also, who is this “us” you’re referring to?
All of that work is eclipsed by the effects she had on Ayaka and Risa's character with all its recrimination, it just pulls the balance all the other way round. Plus, now that that's off the debate, I don't think Hiroko was ever likeable at all, she's just dull like any other rom-com protagonist, with the only difference that it is a lesbian relationship and she's a woman so that's why people in this site still defends her.
And don't get me wrong, I think the backstory was pretty decent and solid, but there's no way I can forget all the things that happened with Ayaka or excuse them just for that reason. And notice it's not even an "I will", it's an "I can", because it's just imposible to erase all we've read until now and all the bad things she did to Ayaka, even with this excuse.
I reaffirm what I said, the backstory would have helped if it was hinted ages ago and narrated earlier, so we could start to understand her reactions, rather than when all the damage is done. And maybe it's fixable for Ayaka, but not for the readers.
And if you're looking for the "us" I mentioned, you can find them in the comments