What happened with this one boy is why most teachers and many parents do not force apologies. When they are insincere, they backfire. Kids think it means "do over!" and that it erases all they have done. And it often emboldens them because they realize that their parents are just making them go through the motions but won't really monitor subsequent behavior. So, unfortunately, your efforts to work with the parents seem to have backfired - 8 other families aren't noticing that your daughter isn't a part of the group, not ever.
I think, in retrospect, walking around with the other parents whose kids are horrible to your child put her in a tough spot. One would hope she would be strong enough to handle that, but she isn't there yet. It's hard to expect that of a 3rd grader. In her mind, she may be asking why you think these people are such good friends, when they raise and supervise (or fail to supervise) kids with totally different values.
I would NOT have her trick or treat with this group! If the chaperoning parents would supervise and ensure kindness, that would be one thing. But it's clear they will not. And if the kids are going unsupervised, well, you already know what will happen to your daughter. I would say to the inviting parent, "Thanks for the invitation, but the kids have made it clear on multiple occasions that they do not like Petunia. So she's going to go with another group." If they protest, say that a discussion has already been held with a few parents, and while they demanded an apology from their child, it made it worse." Do not name the child or family. If pressed, say it doesn't matter and doesn't change the problem. Not one child in the group is standing up for your daughter, and it's kind of a gang mentality now. So your daughter is moving on.
I would connect with a kid or two that she does like, and split the trick or treating into a couple of neighborhoods - so it's not all in your neighborhood where your daughter will run across the kids she doesn't like. And I would start phrasing it that way. Yes, these are kids who don't like her. But as a result of their cruelty and clique formation, she doesn't like THEM. Give her back this power to move on, to reject those who reject her.
And I would cut back your social activities with people you are "close to" but who don't see what's going on or care about it. It's okay for your daughter to see you take a stand. While I don't think all kids in a neighborhood have to get along, and it's fine for 2 or 3 to be special friends with each other, I think it's wrong for parents to see and allow a group of everyone except 1, and think it's okay. In my neighborhood, we have a kid who's a problem. The other kids are told they don't have to seek him out, but if he comes by when they are playing outside, they are not allowed to exclude him. When he starts swearing and screaming insults, one of the other parents will tell him he has to go home until he can control himself and act appropriately. But they never ban him entirely. It's on a case-by-case basis.