Bella doesn't like to be wrong. She likes admitting to being wrong even less.

If I craft my message so that it's less about her being wrong, and more about how a bastard halfblood managed to yoke us to his cause without fulfilling all the promises he made, she will be more receptive.

Think of the wards and other magic that went into disarray after his death. He did not want the Protectorate to survive him, and in fact went to great lengths to ensure that it would not. That she is left cleaning up his mess should enrage rightly enrage her. She need only be reminded of it.