1. You can use him, just remember if you have a very large empire the position of the king affects the loyalty of far off provinces. Its best to keep him, if possible, near the approximate geographical center of your lands. Also if you have him invade overseas, just remember to maintain the contact with your core lands by keeping shipping lanes open.

2. The one and only spears and arbalasters and light cavs combo. Spears protect the arbs, arbs shoot the horde to bits, light cav chases after horde routers. Keep some heavy infantry too, like Chivalric foot knights, varangians, janissaries, billmen for approaching MHC.