IMO it was to make his non-autoattack stuff more appealing. Gathering Storm never seemed worth it, with the small amount of stacks it pulled in.
Yeah, they pretty explicitly themed Infernal Shrines and Battlefields after Diablo, and they released them during a big Diablo event where they released the Butcher and had the Treasure Goblin event.
I don't believe any of the base maps are from Warcraft, they have their own backstory. Interestingly, Hearthstone has referenced Captain Blackheart (there's a Tavern Brawl named "Captain Blackheart's Treasure"), although apparently there's also a WoW NPC who's a pirate named Burgy Blackheart.