You can do pretty much whatever you want if you have the power to supress anyone who disgrees with you. In Alzer, nobles are unchallenged due to the power of the tree, commoners and foreingners are basically at their mercy as long as they are within the sphere of influence of the tree.
The need for the duel was to bind the 5 idiots to the power of the tree, he could still use some power to attack other but not force them in the way he did in this chapter, if he just goes there and starts a fight, the others can defend themselves better, but by borrowing more power from the...
He must know something, he was a loyal servant of Delkira and he was adamant of leaving the DK position vacant, there is no way he picked Iruma out of all humans on a whim and he just happened to have a sort of karmic connection with Delkira.