It was the Scarlet Crusade, not the Forsaken, that invaded Gilneas during the Worgen starter zones to exterminate the evil beast men in the name of the Light.
It was the Forsaken and their Apothacaries, not the Night Elves and their Druids, that came to their aid and helped them tame the Werewolf.
Leading to the Worgen joining the Horde instead of the Alliance.
I’d like to ask Nozdormu why we are are not protecting THAT timeline.
Because gross
Gilneas left the Alliance because Lorderon wasn’t willing to round up all the orcs and execute them all for trying to burn Azeroth to the ground.
I don’t care who attacks them, there is no world where Gilneas sides with the Horde.
Give me the timeline where Gilneas is retaken by purging the remaining forsaken.
It was something I had hoped for waaaay back in WotLK, before the Cata races were leaked. When Arugal had been brought back as a servant of the Scourge and started a cult in that one area in Grizzly Hills, I thought “this could be perfect! We could see in real time how a new scourge wing breaks away and show the same persecution from humans that we’re told the forsaken experienced, except be there as it happens! The forsaken would have another race to relate to, and maybe the worgen could relate to the orcs’ blood fury. And if they shapeshifted, it’d be a way to ‘blend in’ with alliance races incognito for subterfuge.”
Instead, Blizzard just turned the forsaken into the scourge and made the worgen “the actually good monster race” that uncritically integrates into the alliance while Cata made the horde as a whole look worse in every way. I felt absolutely crushed. 
Garrosh might actually approve because of the Lo’gosh/Goldrinn lore. 
coupla years ago, we might have said that there is no world where members of the Burning Legion joined the Alliance.
To be fair so did the elves.
And they didn’t. The Man’ari who joined the Alliance explicitly rejected the Legion.
so the Man’ari who joined the Alliance were never, ever active members of the Legion?
The Legion had already been defeated at that point. Arzaal himself admits the man’ari at that point had basically fallen apart to nothing. I don’t think they’re lying about regretting their actions but that doesn’t change the fact that they didn’t attempt to return to the Exodar until they had nowhere else to go.
Why would the choice of invader push a human settlement towards the Horde? I doubt they like the horde any more than a bunch of religious zealots.
Hypothetically, I think it’s because they wouldn’t be considered human anymore and instead find themselves on the other end of the “humanish race / monster race” divide, like the forsaken.
Would never happen. Gilneas hates the orcs and would never side with the horde.