A few notes -- it would probably be best from a time POV that the lobbying potential Lords to be offscreen and be represented by one big diplomacy check with lots of aid anothers.
Regarding various forms of scouting the enemy (birds, scouting, etc.), the barbarian horde also has a considerable number of special units and other scary surprises. It seems that Trilak the Derstroyer, which is what the demon is calling himself these days, has a number of mighty and disciplinged ogre and fire giant warriors (who double as rock-throwing artillery), some demonically enhanced, commanding the rowdy orcish and goblinoid mass quite effectively.
Their special units are numerous and disciplined. They have hobgoblins with lances and spears mounted on metal-plated horned beasts, a bit bigger and wider than horses, with elephantine legs that can probably trample man-sized soldiers quite effectively. They have larger, less armored, long-necked versions ridden by some giants and ogres, swarms of goblin archers and kobold sorcerors, and manticores with four small missile/magic-users each. The most frightening thing you find is a 30-foot tall (at the shoulder), six legged armor-plated beast with a long neck, firey eyes, and strange circular patterns in its armor plates. Alethia's birds see the horde finishing off a town and see the beast in action. It swallows rubble and rocks and its plates open up and blast out firey boulders like cannon balls. He also has a firey breath weapon, and eye-beams that can seemingly melt almost anytihng.