I can understand the dislike on the new girl, she quite a handful and i am trying to be gentle here but at least things got resolved really quick, but i don't agree with the bit about being bad writing of her just acting now.
As spectators of the story, it may seem that we have access to everything and anything that happens in it but the truth is that many details are just too much to always address, Hanako friendship with the gang solidified pretty recently until then she interacted with Saotome more in secret and in isolated moments, there is a whole bunch of things like studies and personal life that happens and she may have only now got the picture of who in her mind was impeding Hanako of returning to the dojo, it is at most contrivance.
Hanako "dirty past" wasn't so groundbreaking, bullying is serious but it happens all the time at different levels, truth be told i felt that her friend was too weak-willed and i am sorry if i am being offensive to anyone that dealt with the same but having a friend willing to help and capable of doing so but still allowing yourself to be driven at that point was a little too much, but mind you i am not trying to say she is to blame but that she did have people close to help her but she just toke it.
And the author played Hanako self-blame in a well-done manner, as a teen, she overreacted the situation in her mind and assumed that she was to blame too which many people of her age do but things got resolved in a quick and refreshing way, i doubt she already overcame her feelings but as the girls were pretty direct about it much of that fog that clouds her judgment must have already dissipated.