It doesn’t make her a bitch to the end of her days.
No but at this end of this arc yes.She know she had a BF but still act flirty toward Chidori and end up hurting her because she doesn't bring up the BF.
Clueless teenagers fucking up their relationships is novel and unusual how exactly?
I don't exactly see the point ? I didn't said it was unusal but that doesn't mean i must pity her. I don't think it was hard for Igarashi to said she had a BF to Chidori and coming month later claming she is Chidori's ex to one of her friend as a joke but like Nanoha said "Nobody is laughing" and the only excuse of Igarashi is "You were alone so i was just nice", this can be interpreted as "It was your own fault to believe i was into you".
In the end, yes Igarashi was sexually troubled if she love girls or boys but that doesn't change the fact she had hurt deeply Chidori and nearly make her refuse love fromanyone.