I tend to just pick whatever is the newest, with a highly selective memory, trying to tie it all together in a manner that "makes sense" to me, which is why my mileage may vary when professing the factual state of the fluff.
Wow... How can you keep up/keep it all straight? *boggles*
I tend to go with whatever I feel like, and failing that, the oldest fluff. For example, in our take on the setting, the warp isn't a Hell-dimension. It's just a malleable sort of unreality. It's one of the primary reasons its denizens tends to murder-hate sapient realspace critters: realspace things with imagination and feelings impose both on the warp, much to the dismay and the occasional mass-death of its natural inhabitants.