So Rogues aren't 'Rogues' unless they've specifically contract killers? What about our Adventuring Rogues or level 1 Rogues? Why even call the specialization/discipline 'Assassination' if there's no distinction? It's because Rogues occupy a larger niche than gameplay mechanics allow.
Subtlety would be great for thieves or spies, but since WoW is a combat-oriented game, players are given plenty of combat skills. There's a similar problem with how Pirates/Combat rogues preform, but since they're repurposing and renaming specs in Legion, they may do more to differentiate them.
Onto your last point, Priests are more akin to holy healers than shadowy figures, but I'm sure they won't be made unwelcome in their Hall. They'll probably pick/make a location that's a compromise between them. Same thing's happening here- qualifications for a 'Rogue' being good at fighting, stealing, and sneaking. Common thieves are just as much Rogues as Assassins, so their wide definition requires some common ground. Assassin Ravenholdt doesn't need pirates or bandits, so Criminal Underbelly it is.
Teleportation still seems like more of a gameplay/plot ability, rather than something acknowledged much in-universe. If anyone utilized it realistically, then there 'ought to be Goblin mages everyone revolutionizing transportation to turn a quick buck.