I like the idea that every single of Big E decision and origin was to made abstract and never certain in the first place The writter of Master of Mankind novel confirmed this. Even in the novel, the Emperor appeared to be different depending on how the people sees him from mild differences to downright extreme. Even he asked to Ra if he actually speaking or not or he just answering on what Re desired the most in his soul.
I mean,am I following you right that Emps being created as a psychic weapon and the Shaman story being a mythologized version of his birth are kinda saying the same thing?
"Why is Erda even a character?" Because the writers who created Erda wanted to undermine the Emperor as a character by making him look as incompetent and deceitful as possible at all times. Unfortunately for them, their warped world view means they couldn't see how Erda comes across to regular people, not steeped in their ideology. Thus the reaction of basically everyone to Erda, vs their intentions. There has been a systematic campaign to take every achievement of the Emperor away from the character, without any attempt to excuse the failings of the Emperor. (Astarties, the Primarchs, the Great Crusade, the attempt at an Imperial Webway.) It's not just diminishment, (not just saying 'actually there were a lot of people involved in the effort that were forgotten by historians/edited out by religious fanatics.) but deliberate character assassination.