I disagree
Whither would my mantle go? Where is the nearest
vessel now? Some hapless mortal, perhaps,
ignorant of its true nature? The instrument of
some foe of yours, in alliance with me, ready to
steal away the mantle and leave you vulnerable?”
Maeve giggled. “I can play chess too, Mother.
Better now than ever you could. And I am now less
a liability to you alive than dead.”
Molly is not some clueless mortal, she was a pretty good candidate and Maeve was more of a liability to Mab alive than dead, which is why Mab killed her (via Murph).
I do not think the outsiders trusted Maeve enough to tell her that Justine was infected.
I'm not saying that doesn't matter, it does, however here is why Maeve didn't win. While not perfect in many ways, both Ladies, Molly and Sarissa are sane, thus balance is restored. Because the Ladies are sane, hardworking and doing their jobs, the Queens can work together when need be as we saw in Battle Ground.. There is now a united front against the Enemy, there wasn't before.
The Enemy had to go to plan "B," once Maeve died and Justine didn't get the mantle of Lady. Plan B was to undermine the Accords, thus infected Justine's new mission was to get pregnant as HWWB admitted to Harry on the boat at the end of Battle Ground,
that plan came very close to succeeding. Not only in undermining the Accords, but to invade Demonreach itself, if Harry hadn't realized in the last moment that Justine was possessed by an Outsider, they would have penetrated the island defenses and all hell would have broken loose.
Keep in mind, Maeve was raving and just a wee bit insane when she told Mab all of that, and she still was talking about the Lady's mantles. She didn't succeed there, it isn't clear or rather there is no evidence that Maeve even knows that there was a plan "B".