The "blood, bone, and breath" statement, I think, is supposed to parallel, "mind, body, and soul," except it actually just means, "body, body, body." Harry's mind and soul are still his own, as shown in Ghost Story by a certain archangel (and, you know, Harry doing his thing on his own during his soul walkabout).
The lesson from Uriel, which Harry immediately chucks back into Mab's face, is that Mab would have to control his body physically if she wanted to control his every movement, which would obviate his will. That brings me to Cold Days, during which Harry begins to use raw willpower more directly than ever. For example, he broke Mother Winter's "shackles," which were wrought with her own will. I think, at the level Harry is playing now, Mab wouldn't be able to directly puppet Harry anymore, or at least not for long. I think it's one reason Mab went out of her way to manipulate Harry into cooperating with Nicodemus in Skin Game (isolating him, waiting until the last possible moment to tell him about the parasite, etc.) Essentially, Mab makes another deal with Harry to get him to do it (though it probably isn't a Deal in strict Fae terms).
Anyway, I'm not sure about the details of Winter Law and whether or not Harry could find a loophole in the agreement to fulfill the Third Service and weasel out of being the Winter Knight. The only stipulations Harry makes are that his body is restored to health, he's given enough time and power to rescue his daughter and get her to safety, and Mab never commands him to "lift [a] hand against those [he] loves." The exact word "command" might be tricky enough alone to weasel out of the deal. For example, if Mab commands Harry to kill Murphy in vengeance for her part in killing Maeve, Harry might be able to use that as a way out of the deal, even if he doesn't actually kill her.
Mab also might've dealt in bad faith from the start, depending upon the mechanism she used to heal Harry's back. Welching on Winter Law, for some reason, temporarily paralyzed him again; Harry's exact words were "before my service begins," which may mean before he accepts the Winter Knight's Mantle. So he should not need the Mantle to still use his legs. On the other hand, if Mab interpreted it as "before I give you your first command," or something, then it's still technically accurate enough for Fae purposes.
I'm not sure about what else Mab might order Harry to do. Traditionally, in Deal with the Devil tropes, someone will make a bargain in exchange for completing a task that's impossible, and use some alternate win condition to accomplish it.
Personally, I think Harry is just going to get strong enough to change "masks," as VaderrIngle puts it. Harry won't stop being the Winter Knight; he'll simply become something else. Lord of the Little Folk or something, maybe.