When did they say/hint that W&H would pull the whole city into Hell? When was it hinted that the thing that returned inside Cordy's head would summon beast that would try to switch off the sun and plow the whole of LA into darkness ? This is Joss fantasy universe and it has its rules - as well the rules of the genre. Everything is always a surprise, because the audience needs to be kept in suspense. Because of it, I am positive that if Angel decided to ignore the PtB's vision of the Circle - the Apocalypse would have been imminent - if only because W&H got the idea that he is finally 'tamed' and they can get their way with him, his friends and loved ones, and the city/world he is protecting.
Well, I never got the impression that the apocalypse was imminent. Angel just says Cordy's visions 'showed (him) where the real powers are'.
Then say Angel had no idea of how angry the Senior Partners would be. My post above, the quotes I bolded and pasted there, still show that he thought he could never bring down the Partners and was out for a temporary reprieve. Relevant quotes:
'We can bring their gears to a grinding halt, even if it's just for a moment.'
'This isn't a keep-fightin'- the-good-fight kind of deal.'
'We can't bring down the senior partners, but for one bright, shining moment, we can show them that they don't own us.'
It was essentially a suicide mission for his/his team's own gratification.