Well one thought the warp gate or Eldar gate thing on earth would let all the daemons onto terra
I don't think the wards were connected to the Emperor. The Emperor had to seal the gate after Magnus destroyed the wards with his warning.
Magnus would have been able to guide the Astronomican.
One question back at you what time would you think this to be set in the start of the cursade?
The OP stated right before Ullanor. So about 4 years before Istvaan.