My party almost broke the AP. When they got the Forgotten Pharoah low on hit points they stared hitting her for non-lethal. If they would have captured her instead of killing her the AP would break. In the module as written the Ib would not have left the body and book 5 & 6 would not happen.
As it was I had her last remaining cultist attack her thinking that he would be under orders not to let the Forgotten Pharaoh be taken alive and he was able to take out the unconscious Forgotten Pharaoh thus sending the IB to reunite with the BA
That's exactly what my PCs did, which unexpectedly ended the campaign. But I liked it since it was a beautiful way to show that the story changes from the PCs' actions. That makes them feel important. Overriding their actions to get book 5 to trigger irrespectively would have felt like a cheat to me.