Nice thinking, however, when Harry killed Slate on the Stone Table, Mab gained not only her own power, but Slate's blood (and life energy/soul/whatever) too. Lea explained it quite clearly in SK.

Besides being the Winter Knight, Slate was vanilla. Blood has power, but it isn't that significant, IMO. Now, Harry's blood, on the other hand...

That's only the case if Mab has bestowed the mantle on a new Knight, though. Otherwise, Winter is stuck without a mortal agent-- and from what we've seen in the books so far, the Courts tend to find their Knights very useful.

I think this is one of the more important questions about what's been happening since Harry's death: did Mab choose a new Winter Knight or not? If not, Winter is at a disadvantage similar to the one it had while Slate was being tortured. If so, then how would Mab manage to wrangle Harry back into the Winter Knight job?

I don't see it that way; I think that each Court has a limited amount of power (let's call it n), whether it has a Knight or not. The mantle is a chunk of power the Queen loses and the Knight gains (in order to keep the balance. That is, the power level of the Court always equals n). So, if Summer has a Knight and Winter doesn't, Summer has more possibilities to use its power in the mortal realm, but isn't more powerful than Winter.

The source of the imbalances, so far, has been that the mantle of (first) Summer and (later) Winter have been trapped within their respective Knights when they were incapacitated (Lily was stoned and Slate was treed... and I noticed the symbolism there as I was typing this), so the chunk of power the Queens had given their Knights couldn't be used by them nor could it revert back to the respective Court (that is, one of the Courts had a power level of n-1, while the other's was still n).

Now Summer has still a power level of n, but Winter's is n+1 (or whatever power value the athame has).

Well, if Harry is indeed DED dead and the Knight mantle has gone back to Winter, Mab is now more powerful than Titania by one athame.

However, I think that if the balance of the Faerie Courts had gone so far out of whack as to prevent Summer to regain control of the Table, we'd be looking at something a tad more cataclysmic than a foot and a half of snow in May.

How far have you read into the series?  Harry's potential to wield power over Outsiders - and how it came about {to some small degree} - is discussed by Lash/Harry in White Night.

Malcolm Dresden was vanilla mortal.

Malcom was vanilla, no doubt about it. But he might come from some über awesome bloodline.

I get the feeling that Maggie LeFay engineered Harry's birth to the last detail.