"Perhaps the meddling of Blanks somewhere far, far down the line is the reason divination is uncertain in the first place? Little ripples moving through the warp, making different futures possible... "
Here's the problem with that theory besides the complete lack of basis in the fluff; everything is ultimately affected and essentially 'tainted' by seminal events and chains of causality the Warp can't perceive, which means that everything is wildly uncertain, and the Warp fails to be at all useful as a predictive engine. Taking Praetus' logic to its ultimate conclusion, nothing the Blank does or is ultimately responsible for in terms of causation can ever be accounted for; this doesn't result in 'little ripples that make different futures possible' it results in full blown impenetrable chaos that invalidates Divination.