Restating the proposed scenario, in light of the discussion we've had, integrating some of Griffyn's ideas:
Let's imagine a scene, at the beginning of Small Favor. Mab has entered Harry's mind, and is approached by Id Harry, who has the relatively recent task of watching over the newborn Bonnie. And Id Harry and Mab begin to deal. IH wants Harry's immediate survival, protection for Bonnie, and a chance to get laid. Mab wants Harry as Winter Knight, and also wants to earn Harry yet more power. Mab sees someone she can work with. So, the bargain is struck, and a relationship begun.
Id Harry will guide Harry ever closer to taking Mab up on her offer of Winter Knighthood. This will aid in Harry's survival and further amass power to help protect Bonnie. Plus, being WK definitely ups the chances that Harry will finally have some sex. More generally, the WK mantle will increase IH's power over what Harry does, as it encourages Harry to pursue his more animalistic, base tendencies. To enable IH to help further with these goals, Mab will give IH the ability to hide Harry's memories from himself. IH is responsible for hiding Harry's fire magic for most of SmF, at Mab's behest as part of this deal.
On Demonreach, during the final battle of SmF, IH sees an opportunity to win further favor with Mab. Before Michael goes up into the helicopter, IH guides Harry's hand during a distracted moment, similar to when Harry picked up the denarius instead of Harry Carpenter (DM), or when Mab made Harry stab himself (SK). IH take's Thorned Namshiel's hand from Michael's pouch, and stows it away in one of Harry's pockets, unbeknownst to the conscious Harry.
At the conclusion of SmF, in the hospital chapel, Mab enters Harry's head again. IH presents his gift to Mab, the fallen who assaulted her castle. He is rewarded with a Winter boon, a fine-wrought silver snowflake pin, analogous to the leaf pin Harry received from Summer for his efforts in PG. IH is wearing this pin on his lapel next time we see him, in SG. With Demonreach in the discussion, IH mentions his sight visions to Mab. They further their bargain, adding the goal of guiding Harry becoming the Warden of Demonreach, newly within Harry's capability thanks to the gift of soulfire.
In TC, IH and Mab coordinate their efforts to assure that Harry moves towards the island. IH prevents Harry from confronting Shagnasty directly, by blocking out Little Chicago at a crucial moment and making sure Bonnie triggers a headache when Harry goes for Mouse. Instead he puts the idea in his head to take Morgan to Demonreach, and to perform a Sanctum Invocation. He is Harry's intuition, after all. Mab plays her part here as well, intimidating Bob into silence over why Harry isn't using LC. Since Lea was still a Sidhe-sicle, Mab could come and go as she pleased from the Harry's lab during TC, through Lea's garden, assuming she meant no harm. And Harry only goes into the lab the one time in that whole case. Plus, Mab foresees a side benefit of terrifying the little air spirit; he'll ask Harry to prepare a second vessel as a bolt hole, which will then be in place for when Bonnie is born. And since Mab knows Harry has access to Bob, she can be confident in CD that he has access to the info on how to kill Maeve. Furthermore, Mab helps guide Molly towards the skill level and power she will need to perform the "delivery" of Bonnie.