There is something actually that it was never discussed properly and it is the sacrifice.
In order to unlock the Orb a sacrifice is necessary?
The short answer: no one is sure. All that really seemed to necessary was power, and so while a sacrifice could be one method of obtaining that power, it by far not the only method.
Most likely Corypheaus simply wanted to go with the obtain-power-by-blood-sacrifice route because that was his preference. That it was the Divine he had chosen was most likely because she was the head of a religion that directly had a hand in the death of Old God worship, and because he knew that such a figure dying would throw the world into chaos.
But everyone is free to speculate.
The only time we get insight as to how may be activated was in this conversation after trespasser:
Inquisitor: You gave your orb to Corypheaus?
Solas: Not directly. My agents allowed the Venatori to locate it. The orb had built up magical energy while I lay unconscious for a millennia. I was not powerful enough to open it.
Solas: The plan was for Corypheaus to unlock it, and for the resulting explosion to kill him. Then I would claim the orb.
Solas: I did not foresee a Tevinter Magister having learned the secret of effective immortality.
The only reason I don't think a sacrifice was mandatory was because Solas could have done it himself, if that was the case. It may not have been easy, but it could have been done. If Solas can muster up forces/intel to infiltrate a powerful Inquisition and put a damper in Qunari operations, then access to the Divine could have been achievable (if he had the mind to). Corypheaus could do it with a handful of wardens and a small force of tevinter extremists, I don't see why Solas and his agents could not do the same.