I think you are on to something, but haven't followed your idea to the logical conclusion. Mab suspected that she would die, so would have planned for it.
She knew that Molly couldn't cope as Queen, warning Dresden to kill Molly: so would have set things up so that Lara would become Lady when Molly was promoted, giving Lara time to get a hang on Winter before the enforced celibacy turned Lara insane - crazy Lara would then be in a position to kill Molly and take the job of Queen.
Mab would only warn Dresden so that he could plan a 'die for a few minutes' trick on Molly, allowing Molly to revert to mortal, and thus be in a position to become Lady again with Lara's promotion.