I think that in many ways the Winter Knight gig isn't going to be as bad (from a fan perspective) as we have been led to believe. Of course it will be a tool to place more restrictions on Harry and put him in difficult situations and so forth, but he's also received some significant upgrades to ratchet up his badass factor.

As for working for the queen of the evil faeries - good. Give me goblins and ogres and fetches over spites and nymphs and twinkly bullshit any day. It's like a choice between being Edward Cullen or Spike, and Harry got Spike. And there is also the pattern of behaviors exhibited by Mab so far. Except for her ongoing quarrel with the Summer Court, most of her visible actions have been concentrated on opposing the ploys of the Black Council, Harry's arch-enemies.
- She shows up to advise Harry regarding the Heirs of Kessler, who included the only known member of the Black Council. Yes, she gives reasons for it, but the Queen of Air & Darkness almost certainly has other minions of comparable ability she could dispatch as acceptable substitutes for Lea. She was there because she was taking an interest in Harry.
- She played some behind the scenes stuff during the crises involving Molly's emergence as a wizard, and seems to have manipulated events (i.e. feigning indifference to attacks and intrusions, having her elite troops kidnap Molly and bring her right to Mab's HQ) to bring Harry to Arctis Tor and do what he did to either witness more results of the Black Council's schemes, and/or hook him up with Molly for the teacher-benefits things that Michael alluded to and that we have seen with his own abilities which have benefited as a result of his teacher role.
- She worked on curing the Leanansidhe of the bad influences of Bianca's gift; Bianca's party/promotion almost certainly being another event with Black Council fingerprints on it.
- She used Harry to stop Aurora's plan, which Harry later will number among the suspicious events that have been incited by the Black Council.
- She uses her second favor from Harry to pit him against the Denarians, whose higher-ranking members seem to include a Black Council member or ally. Also their attack on Marcone is suggestive of a Black Council plot. The whole thing is suggested to be a reaction to an ordinary "vanilla" mortal overstepping his bounds by becoming a signatory to the Accords, but we have seen in the past that the Denarians don't give a damn about the Accords and will freely break them whenever they feel like it. On the other hand, Marcone became a signatory as his reward for helping to foil a BC plot - namely the White Court coup & genocide strategy. Also note who enabled Marcone's rise to Accords staus - two of his sponsors are the only two known members of Ebenezer's Gray Council - Donar Vaterrung & Harry Dresden - and thus committed opponents of the Black Council. The BC almost certainly nudged the Denarians to go after Marcone.

Later Mab outright states to Harry that she wants a Winter Knight who is motivated on his own to go after her targets, when she tells him that her backup candidate was Thomas, since he'd be eager to avenge Harry. Plainly she wants Harry because Harry is already motivated or naturally inclined to prosecute the fight against her foes, whether he realizes it or not. Since he's friends with the new Summer Lady and Summer Knight, and has no grudge against Titania, plainly she has different targets in mind for him.

In short, Harry as Winter Knight is not Harry being corrupted and dragged away from his main agenda, it is rather Harry being suped up and prepped to take down his own enemies. I'm rather looking forward to that.

As for the supposed unseasonal weather, the simple explanation for that is Harry's death. Recall the issues with the Summer-Winter imbalance that the Ladies explained to Harry back in Proven Guilty - since Mab was keeping the old Winter Knight alive but out of play, the power of the Winter Knight was effectively missing from the equation, so Mab & Winter were weakened against Summer. By killing Slate and passing the power to Harry, the imbalance was redressed, and once Harry died, that power snapped back to Winter, if I recall the explanation of Ruel's death from Summer Knight correctly. Since the power of the Knight is definitely separated from his queens, as proven by the imbalance of having a live but inactive Knight, that means that with NO Winter Knight, his power is ADDED to Winter, and now the imbalance goes the other way. With Slate & Ruel active, the sides were balanced. With Ruel dead, the balance went to Winter. With Fix activated, the balance was restored, but then with Slate imprisoned, the balance tilted to Summer. Once Harry became the knight, there were now two active knights, so they balanced, for about a day, but with Harry dead, his power is returned to the Winter Court so they are more powerful than Summer, who has a significant portion of their power tied up in a changeling mechanic.