We know that Mab doesn't like to act overtly, and that she likes to kill multiple birds with one stone. With this in mind let's take a look at the unfortunate incident with Carlos and Molly. In Cold Case, Molly goes on her first mission as winter lady to Alaska. Her mission is to collect draftees from the Miksani. They haven’t been able to send the conscripts because a cult has been abducting them.
She runs into Carlos and they take care of the cultist and save the conscripts. Afterwards, they attempt to have sex with some disastrous results. Molly blacks out, the mantle takes over and almost kills Carlos. The winter lady can't become a mother, so the mantle didn't like it when she tried to get laid.
Carlos is based in California, so Alaska is a bit of hike. I'm sure winter's intelligence network monitors the movements of the council's upper level wardens. Of all the places Mab could've sent Molly why send her to a place where she'll run into Carlos? Does anyone really buy that Mab didn’t know he was there.
Mab has a certain degree of foresight and she isn’t stupid. She knew what would happen between the two of them. What does Mab get out of it? I think she gains a lot out of it. It makes Molly take her new role seriously and respect Winter law. Harry and Molly have never been on great terms with the council, and they might be pissed that the two of them are on team Winter. Molly just made a first impression as winter lady to the council, she can take one of there most well respected wardens and cripple him without even trying. Maybe that would give the council a reason to think twice about taking action against Harry and Molly. The incident will also side line one of Harry’s allies. The less people he can go to for help, the more he has to rely on Mab.
The timing of this was also perfect because Molly was new to being the winter lady, so nothing suggests Mab’s culpability. It’s not her fault that the new person didn’t finish ready the mantle’s owner’s manual. The council would be less likely to believe this was an accident if Molly had been on the job longer.