It seems to me that Mab has probably more than one or two reasons for wanting to marry Harry to Lara.
I'd argue, given what we've seen in recent books, that Mab always tries to protect Harry (despite Harry's objections that she would only care for a Knight who can protect himself, she's been doing a lot to help him or prevent him from doing mistakes). So I assume that her main reason for her to make them marry is to protect Harry. She wanted it to happen right away. Why? Most people thought it was to torture Harry, but what had just happened? Harry had just been kicked off the WC, so he's basically defenseless. An alliance/marriage with Lara would mean one thing to Harry's potential enemies: if you attack him, you attack the WCV. Mab cannot protect her Knight openly, or it would make him look weak, but by giving him an ally who is basically a Chief of State of an Accorded Nation, attacking Harry would mean a lot more to potential random enemies (it's not gonna stop Mavra or some crazies/really powerful enemies, but it can stop others).
Second, Mab has to repay the favors she owes to Lara, and from her point of view, there's no reason not to strengthen Winter's position by making a more permanent and reliable ally out of the WCV, especially since Lara is such a good political player.
Third, the marriage is meant to piss off Molly, and to basically tell her to forget her human emotions and to start acting like a Queen of Winter. Mab said she was lacking and too dangerous to become Mab right now. Considering what we've seen about Mab, it's probably about being "cold" enough to put your feelings aside and do what's necessary. And Molly can't say no to Harry and will always run to help him, even if it means somewhat endangering her duties as a Winter Queen or exhausting herself and leaving her vulnerable.
Fourth, it widens the gap between Harry and the WC. From what we've seen at the end of BG, the Accorded Nations don't trust the WC, or they would have invited them to their private talks. One-Eye and Mab at least know that they're probably at least somewhat directed by either Nemesis or the Black Council (or both). It also protects Harry from the WC, because basically killing Harry would mean waging war on two Accorded Nations, which even the WC might be reluctant to do, especially in their weakened state.
Fifth, it protects Harry from Lara. She might be a "good guy" nowadays, but she still puts her interests or that of her family/Court first. Making Harry part of that family would make her less likely to kill him for political reasons.
Also, I agree that Lara totally knew what she was asking. She's completely in femme fatale falling for the detective mode right now. Freydis told us as much in PT.