Here are some relevant quotes from SK. First from when Harry is questioning the Mothers.
I held up a hand. "Hold it, hold it. Look, as I understand it, this power of the Summer Knight, his mantle, it can't just exist on its own. It has to be inside a vessel."
"Yes," Winter murmured. "Within one of the Queens, or within the Knight."
Then a few paragraphs later:
"How does the mantle pass on from one Knight to the next?"
Mother Summer smiled, but the expression was a grim one. "It returns to the nearest reflection of itself. To the nearest vessel of Summer. She, in turn, chooses the next Knight."
I know that these specifically refer to Summer, but for the sake of argument and in absence of other evidence I am going to proceed with the notion that the same applies to Winter. From these quotes we can glean a few things. Number 1, the mantles return to the nearest Queen. Number 2, that Queen then bestows the mantle onto the knight. Number 3, Mother Summer makes no mention of the previous knight having to die for the mantle to be released. I personally believe that there is a way for the Queens to take the mantle from a knight without requiring his or her death. Finally, no mention is made of needing the stone table for the transfer of the mantle. This is further corroborated by the fact that Fix was made the Summer Knight after the Stone Table was in possession of Winter. Also corroborated by logic because if the stone table was needed then the potential exists for the Knight mantle to be stuck within a Queen for months while they waited for the stone table to come back into their possession. This would result in an imbalance between the courts since one side does not have a Knight exerting their power.
Therefore, if we accept that the Knights Mantle could have been removed at any time and held by Mab and then bestowed onto Harry at a time of his choosing, why did she keep Slate alive (besides revenge) and utilize the stone table? My theory stems from two more quotes from a conversation in SK between Lea and Harry.
I shook my fingers and looked at my godmother. "Let me get this straight. Blood spilled onto the Table turns into power for whoever holds it. Summer now. But Winter, after tomorrow night."
Followed by:
She frowned at the Table, then began pacing around it, slowly, clockwise, her eyes never leaving me. "The Table is not merely a repository for energy, child. It is a conduit. Blood spilled upon its surface takes more than merely life with it."
"Power," I said. I frowned and folded my arms, watching her. "So if, for instance, a wizard's blood spilled there ..."
She smiled. "Great power would come of it. Mortal life, mortal magic, drawn into the hands of whichever Queen ruled the Table."
My theory is twofold (because Mab never does just one thing when she can accomplish more). First, Mab did not want to just pass the same mantle onto Harry. She wanted to purify or slightly change it. By killing Slate on the table the mantle was transferred to Winter as power. Mab then took that pure power (possibly without the taint of Slate or Maeve) and gave it to Harry as the Knight mantle. Second, killing the mortal on the table also yielded some additional power. Not as much as killing Harry or another wizard, but I am guessing that their was a power gain regardless. If this was then added to the Mantle during the transfer it could possibly give Harry an additional power boost to make him greater then Summer and better able to face what is coming.
