I can't say I know what way Molly can leave her Winter mantle, can't tell which Lady quit her job & survived.
And as for lies, just look at the end of GS. She technically lies to Harry because she thinks she can control him, deceive him. She could just as easily tell Molly to lose the humanity to be a better Queen because she knows that it will be a quick move to be more powerful. But she also knows her Knight and that her Lady is just like her Knight.
If we're going with technicalities here, in the same Ghost Story's chapter Harry pointed out that technically Uriel manipulated him into a gambit with the risk of Harry losing his literal, actual, soul. So why do Mab gets labelled as Bad when she had been proven more forthcoming whenever she said something to Harry or Molly than Uriel, who won't even admit anything to Harry? When have she reneged on what she said she will do to/for Harry, even he remarked that Mab keeps her words.
So by telling her a partial truth, she may be pushing her to be a better Lady than her own daughter by being herself. Half truth with the fae are not lies, just not the whole truth. And yes, Molly becoming Queen so soon wouldn't be good for her. But Molly the Queen after getting a good hold of her abilities and keeping her humanity may be what Winter needs, and Mab may be jealous of that because she went through becoming Queen and not having time to master the balance as Lady.
nah I don't buy it, it sounds very much like conjecture to me, I mean unless there's more than twice of interactions/action and/or there are inconsistencies in the (behavioural) pattern, I don't make guesses on what a character might be up to; and for Mab this is very much her standard pattern, a consistent & repeated pattern, too. Is it possible? Yes of course anything is possible, after all Jim still considers her a villain, but there's nothing solid to back it up.