And I disagree that Mab didn't "heal" his back injury. I'm assuming you are referring to the part in CD where he says "Screw Winter's Rules" and suddenly felt himself paralyzed again. I don't think she's making his body "forget" that it's got a severed spinal cord. She actually healed it. It's been established that the Fae can do real healing on those they have power over. Leah did it to Harry in Grave Peril, and there is no reason to think Mab would be less capable. Also it was worded in Harry's agreement to her that she heal his body, and Mab is a stickler for the letter of the agreements she makes. Besides, his body wouldn't keep working normally with a severed spine, it would still need to be repaired before it would function for him. It's more likely (to me) that his sudden paralysis in that scene was Mab's way of reminding him to obey her rules, that she owned him, and he better not forget it.
As far as I understood Harry's conversation with Butters, there is a big problem with Mab's interfering with Harry's natural feelings of his own body. Harry doesn't feel pain, injuries, broken spines, ... but they are there nontheless. I am not sure that Mab would be able to really "heal" him -- healing is more a power of Summer. But Winter can numb you down very effectively until your body is able to heal itself.
By the way: Very interesting that Winter is not able to stop the headaches caused by the parasite. All other pains are repelled.