Well the nids are avoiding the sphere by some light years I'd say, anyway, before the imperium is to do anything they should study the sphere, how thick are its walls, what is inside the sphere (maybe the damn outsider has some tombworlds inside), how the c'tan is sustaining itself, etc.
My best advice? get the remaining BF at ANY cost and give the c'tan the welcome he deserves, or either make him weak, wait for his backstabbing brothers to arrive and you have three c'tans to shot, not an easy feat but with an eldar-imperium alliance this could be done.
Of course that would be in my wildest dreams.