If the Emperor was the combined souls of those Shamans in antiquity, and it is normal that when the body dies the souls are released to the Warp, would the Astronomicon be directing that 'release' in a direct way?
And the sacrifices, could they not be a 'top up'? And haven't the numbers of psykers increased in human population disproportionately since M30?
Wasn't there a picture of the golden throne at the start of one of the older rule books where the thing say in it is basicly a skeleton with some ragged bits of flesh hanging off of it?
The Rogue Trader picture had a bit of flesh on the bone, but he did look a lot more skeletal than a healthy person would. I don't remember if both arms were completely attached or not either.