Why were the Centaurn forces unable to defeat the Warhordes with their Bows at the Battle of Salaquin?

The Centaurs their hooves slowed down by sand, vision blocked by swirling dust and lighting crashing down from the heavens were not as fluid in such large, bulky formations.

How do the Dwarven Mercenaries move about on the Surface?

There are several ways for Dwarves to move about on the surface. They can travel inside their war machines, they can move about through tunnels that crisscross underground and or they can wait for night to come. Dwarves can also travel just beneath the surface of the ground, using their powerful hands to dig rapidly.

Why did the Ogrekan invade Orkeylium?

The Ogrekan/Ogres were once the followers of Lord Sezrath until they realised the path Sezrath was taking them down. Fearing the same would happen to the Orcs, the Ogrekan decided to quickly eliminate this threat, before it spread across Coroleya.

During the Battle for the Mountain Pass, where the Orcs first lost their senses to the Bloodlust and Yearning. Why were the Orcs not defeated, while they were distracted feeding upon the dead during the battle?

The Bloodlust and Yearning, works differently for each Orc. For some the need is insatiable, for others the need is more controllable. In either case the battle would still have been won, because Berserkers are less effected by the Yearning and Bloodlust due to their transformation.

If Orcs can only sleep beneath the sky, who guards the fortresses and outposts?

During their campaigns in Orkeylium, Kahloon, and Caldasher, the Orcs acquired a great number of followers that have chosen to swear their loyalty to Lord Sezrath (Goblins, Dwarves, Ogres and Trolls). These believers of Lord Sezrath have been given the task of securing the fortresses that dot their growing empire and guard their supply routes.

Why don't we see the other races conquered by Orcs fighting in the Warhorde?

The Orcs see themselves as supreme beings that have been given the Will of Sezrath to hold and protect. It is a role they jealously guard for themselves. Allowing another race to fight for Lord Sezrath would be an insult.

How could the Orcs not know for so long that they were eating meat taken from the dead on the Battlefield?

To answer simply they did know. They just didn't want to admit to themselves what they were doing. After all, what choice did they really have? It was either eat the meat they were given or die of starvation. Not much of a choice.

What happened to the Orcs that refused to continue eating meat?

That I will have to leave for a later tale.

Why did the Orcs invade other lands?

The greatest desire of Lord Sezrath is to be spread throughout the whole of Coroleya; he wishes to be worshiped and feared by all. This also means raining down retribution on those that have stopped following the Will of Sezrath to worship other gods besides him.