While the scenario to get mag'har orcs from draenor hasn't happened yet, one thing that seems to be assured based on the story is then events back on AU draenor happens around 30 years later from the events we underwent when we were last there.
How does this work? Best i can assume is when garrosh originally went to this au time it was due to the bronze Dragon whos name i forget opening not just an alternative reality portal but time displaced one too.
So now this current time we as horde to go get the mag'har it timeline of au draenor is now running parallel with azeroths MU timeline?
This is why blizzard shouldn't do time travel stories, their always a mess.