One possible reason why Orthodox factions can build the Royal Court is that if they conquer a catholic or muslim province, it doesn't get razed (of course royal estates or baronial courts still would be)

Manco Capac (interesting name btw, could you explain it to me? Is it an actual person): maybe you could have different mounted sergeants for the different eras. The vanilla mounted sergeants would be early only, after that you can retrain them into the high version wich would use the model and skin of Feudal Knights (perhaps with some changes)