Back then, Twilight Ridge did not exist, it was only made when Magtheridon came into Outland. At this time, it would just be normal mountains.
Also, Mag'har were uncorrupted because they were actually orcs who suffered from a disease called Red Pox, so they were put into quarantine. This means they never encountered the demons and such, so they never got corrupted. When Kilrogg approached them, he thought they were all weak and did not add them to the Horde. Though, the Mag'har wouldn't fight their own brothers, even if they were corrupted. That's why I said tall walls, because they were actually quarantined.
Also, Halaa and Telaar don't really have to be that big. Remember, pretty much every Draenei thing in this game is uninhabited ruins, because all living Draenei are in hiding.
Also, I forgot if I added it or not, but Shadowmoon Valley does have the Hand of Gul'dan at it's center. Even by this time, it was corrupted like it is present-day because of Gul'dan.