This roll pretty much assumes that everyone present is fight / doing something all at the same time.
I would require that everyone who wants to be involved or is stuck being involved in the chaotic fighting has a choice to make. If they are running around with another pc, maybe they choose to let his roll decide for them, and thus try to help them out and suffer whatever consequences come his way. However, if you have say three PCs all interacting differently, I would make them all roll as part of joining into the chaos. They all roll, and they each are hit by the collective effects of the chaos.
For damage mitigation there are many ways to do it, but I assume the most reasonable is that if one person lowers the damage of everyone, cool. If someone else does too, awesome, but it doesn't stack--although it will also effect the other pc as well. Just as the first pcs lowering damage effected them. That being said, you can also cause the mob to be more violent, so I would suggest that 2 votes to lower and 1 to raise, provides a 1 to 1 counter, and 1 still keeping the violence down. That could be one reason it is important. Lowering the violence more then one step doesn't sound reasonable to me.
The mob should deal out harm once, at the same time, to all participants, just as the participants also deal in that moment and not before. If someone choose not to roll, but they were in the mob, they will also be subjected to the general harm rating of the fighting, but will not be able to add to it themselves. So that's more of a one sided harm because the area is dangerous.
Lastly, If you find that all the pcs are always involved and there are never any good options for people to choose from, maybe "bonus" lowering / raising of harm means they can pick someone out of the mob to protect or direct violence towards. I don't currently have a copy of the move to compare however, so I'm relying on memory and your words. :)
As for the one action, I would say it is hard to do things in chaos. But yeah, choosing that option should be they have the opportunity to do so without needing to act under fire or any of that. As for how much time between rolls, that depends on the fiction I'd think.