>>2 True understanding here entails a recognition of the girl's predicament, as well. OP needs to tell the best friend about what happened (Including her story of attraction and OP's own feelings.), and to start with admitting that they kissed, but doesn't need to fire at the girl like she's a reasonable scapegoat. There's no such thing as a reasonable scapegoat.
OP, you're probably not going to have a friend after this, though you might work it out over time. As far as that goes: Deal with it.
As for the girl, I do have to agree that she clearly isn't mature, and that is enough reason to not have anything more to do with her. Her immaturity will affect you negatively, and you don't need that.