As a note, has it been mentioned who is the audience? For all that we know, Yonis might be talking to local Holders. Well, some commoners get to hear him, too, but that's just the cherry on top of the thing. Good morale and stuff, less likely to revolt should the Holders be more loyal to him than the Empress.
And on Yonis dying: If the tour is successful, he'd be a martyr. I assume that he's wise enough to have named a heir well in advance. His death would just mean that Ardishapur would become a lot more powerful, with a new Heir getting a big boost on the "royal martyr". Admitted, it's possible that the younger Ardishapur would be some kind of a loonie, but I'd say that it's a given that he's religious. (After all, well, Ardishapur.).