Medivh made a few offhand comments about Khadgar and his power and how he wasnt guardian....yet.
And without the sacking of stormwind and any mention of the Shadow Council, Garona killing the king at the gate makes the most sense, She was conflicted and sad in the game and this leads to her same feelings and station within the Orcs.
As for Sargeras not being named, this movie had to be planned with sequels in mind. Medivh turns into a demon at the end and we had a Naaru in Dalaran. The movie had already introduced half a dozen orcs and another half dozen human characters and it could have named a lot more, adding another layer beyond that probably would have been too much.
I haven't played Warcraft in ages, but wasnt the game basically Orcs and Humans, the end? And doesn't it get retconned every time Blizzard references it? If they are planning a series, changing things around to make Lothar more important to the Orcs, leaving open who the Demons and the source of the Fel are, Making Gul'dan openly leading the orc with fear isntead of as a puppet master, etc feel like reasonable choices.
I wish the human scenes were better in general, but this was a pretty decent attempt at this movie I think, and I would be interested in seeing more