I'd do some combination of the below.

Go on the offensive, call out her past false accusations, her gossiping and spreading rumors. If the other adults feel similar to you, it may get her run out or at least to shut up. I'd do this by talking to the other adults one-on-one or in small groups and talking about how your two friends have been impacted by her behavior. If she ends up accusing you after this, it will look like she's trying to get petty revenge and thus be less believable.

Ignore her, stop talking to her, make it clear that you don't like her and aren't going to go along with her rumor mongering. You aren't going to convince someone like this to take your side, so (like with talking to a cop) anything you say is only going to hurt you. So don't say anything to her, can combine this with the above to some degree by making it known you don't like her. Like above, if it's obvious you don't like her/don't get along, any accusation she makes will be less believable.

If she comes along questioning you, don't answer. Get the kids involved if possible have them complain to parents that she was bothering your group and wouldn't leave you in peace. Letting her cross-examine you gives her the feeling of power and she feels like you are guilty because you are letting it happen. Don't put up with it.

I tend to be pretty non-confrontational, but I'm willing to confront when I feel like it's needed and it seems like in your case it is. She either needs to quit with her behavior or be made uncomfortable enough that she leaves the group.