I don't know if their is any basis to my thought process but to me the initiative steps are global rather than per combat. I always assumed we went combat by combat for simplicity sake, but I feel like with an ynnari list you have to resolve each I step in every combat in descending order.
Thus HoW for each unit, then you move to I9 and every combat with an I9 happens. Then I8, I7, so on and so forth.
For Khaela Mensha Khaine!