No, she's not. The only thing she's innocent of is kidnapping Taiga - she very much did everything else we'd already resolved to take her down for. We pointed out in this very chapter that she'd already killed innocent people, and her response was "yeah well it seemed like a good idea at the time".
Everyone saying BB will just lie about the deal is, I think, misunderstanding the nature of her cruelty. She can be deceptive and misleading, but offering such a direct choice and then simply ignoring the outcome is typically beneath her. It's boring, and suggests she doesn't have the ability to guide the target to the decision she "wanted".
More pressingly, they're trying to make it into a non-choice. "Oh, it doesn't matter if we pick Rin over Taiga, because Taiga will die anyway". No, she won't. By that logic we might as well say "It doesn't matter if we pick Taiga over Rin, because Rin will survive anyway". Own the fact that you're willing to throw away our sister's life just to avoid following through on a decision Shirou already (quite rightly) made, and argue for that, not the outcome that you've invented.
Yes, which Rin very much is.