Yeah, given the information in CD that Fae can ignore thresholds when coming in for benevolent purposes, I think it probably was Mab -- specifically, Mab acting in Lea's place (since she was preventing Lea from fulfilling her obligations), and coming in from the NN through Lea's garden.
As for the Calvin and Hobbes book, I prefer the theory that it was a subtle hint about the hobs that were about to show up. Since they're Winter, that also points to Mab, IMO. (And against future-Harry, since it didn't actually end up being a helpful hint, and future-Harry would have known that and not bothered.)