1) Don't we have TWO eyes...The eye of Anu, the world stone, was one of them?
2) What if the Demons full-on invaded Sanctuary due to breaking open Mount Arreat. What if Imperious wont save the Nephlam from the large scale invasion? This causes Tyrael to fight with Imperious, take his spear, and break open the Diamond gates to FORCE the angels to help the Nephlam since they are exposed now as well? I think that fits all the criteria listed above.
3) I personally do not think we will fight Imperious. Imperious will fall to Diablo in the end defending heaven and Sanctuary and we will finish off Diablo with the help of Tyrael.
4) Diablo has two heads on his(her?) shoulders, likely being Mephisto and Baal transformed into Diablo by the Black Soulstone. There are 3 prime evils. This would be the closest form to the original prime evil, Tathamet, who had seven heads. Also, of all the 7 evils, Diablo resembles Tathamet the most.