They didn't forgive arvo, but they also didn't think that beating and berating him constantly like Kenny was doing was right either. Him being pissed at arvo wasn't the problem, it was what he was doing because of his anger that was the problem
She blamed Kenny probably because everything got disrupted by arvo attempting to run from Kenny and Kenny running after him
They didn't outright forgive him yes, but they don't seem to take much issue with the attack afterwards. You could say they're trying to be more human compared to the other monsters out there I guess. Despite that though, directly after the attack Mike's chewing Kenny out even though Mike himself was shot mere minutes ago due to Arvo's actions. You'd think the group would be more peeved, especially when just before the attack they were freezing and just had one of their own die and are stuck with a newborn with little food. I'm all up for the group warming up overtime to a previous threat, but the way it happens here seems like just an excuse to say "Kenny's bad".
As for Bonnie's comment, she's in the wrong. Luke would've been screwed even if everyone was calm. Worse yet, it was specifically stated that everyone should spread out to keep the ice from cracking, yet Clem, Bonnie and Luke are all in close proximity to each other. Kenny had nothing to do with that. They were close to the house, but not actually at it. What if Arvo got a gun or something, can't take the risk. Even with his limp, something could still happen. Luke died because Bonnie wouldn't listen. That was still Bonnie's fault. Bonus points if you don't help Luke, in which she blames Clem as well.
Anyway I'm pretty sure we spoiled most of S2 for anyone who hasn't played it yet. I think we should just agree to disagree on this since we've both made our points and I'm not sure how long a discussion like this can go on for.