So basically, what I’m proposing is…what if Odium killed the Almighty BEFORE the founding of the Ten Orders and the Knights Radiant?
I don't think the Almighty was killed before founding the Ten Orders. In the last vision the Almighty says that everything that has been shown he had seen himself (implying that he was alive). This includes numerous visions of the Knights Radiant. The earlier visions were also a lot clearer, with specific people and events.
The last vision, however, was what he SUSPECTED would happen, using his limited vision of the future. Thus the image was not so clear, just a city and an incoming wall of shadow. So I think that the Almighty was killed maybe during Aharietiam (the last desolation), which would explain why there hadn't been a desolation since then.