Alliance recaptures Northwatch already at the end of Tides of War though.
True. It's one of the biggest flaws of "super amazing Horde civil war" storylines. I.e. that the "right" Horde members face no repercussions for their actions from the "wrong" ones even when those actions are taken long before there's an actual civil war and the "right" ones are protected by armies loyal to them and there's no reason for the "wrong" ones not to punish them. To think of it, this would fall under plot armor.
Just happened to have my copy of Before the Storm on the relevant page from my last browsing. At the start of chapter 28, when the Alliance arrives at Stromgarde and Gathering is about to start, Genn tells Anduin that Gilneas is home to "them" while pointing at Thoradin's Wall, where Sylvanas and the rest of the Forsaken are stationed. As for the blockade, I can't seem to find it right now. It was at the start of Legion, on how the Forsaken fleet was sent to fight the Legion and as such was pulled from its blockade. Though I think it was of the entire Lordaeron and not just Gilneas. Maybe @
Friendlyimmolation will recall the specific quest.