I think they might have had two plans going. It's the only way to explain Lillith trying to kill him later one. My thoughts are right now they are actually trying to run a rebellion in Hell and Azazel actually is trying to get the best candidate.

If getting demons to do things is like herding cats, it kinda makes sense. Even the higher ups are more focused on their own personal gain than the big picture. Since it takes two to tango and pull Armageddon off and the demons weren't talking to the angels about when it should happen, they figured *shrug* what the hell? Maybe Armageddon seemed like a pipe dream that would work itself out if it was meant to happen. Which it did.