Yep, we'll just have to disagree on the empires co-existing thing.
Re the warp pre Slaanesh, the problem is it wasn't that hellish landscape. Firstly, we know the eldar were able to go into it practically at will - when they died, they just reincarnated after their souls went into the warp. Secondly, warp travel was safe and not at all the dangerous, Event Horizon esq issue it is now. Humanity travelled the entir galaxy using warp drives without the need of the Astromicon or the Emperor, and did it safely and without ever encountering hordes of demons or the chaos gods. Thirdly, there weren't demons and chaos gods manipulating the universe for their own event - the eldar didn't even know of the original three chaos gods at the height of their power. Clearly, they were there, and maybe they didn't coalesce in the same dramatic way Slaanesh did - but they were clearly also far weaker than they are now, and the warp was a far safer place. Even the original lore about the emperor claims that he was borne in response to the growing darkness in the warp that was to come in the future, not because the warp was already a hellscape. Everything in the lore suggests that pre-Slaanesh, the warp was far safer and the chaos gods far less powerful than they are now, and that post Slaanesh things changed. Whether that's temporary or a permanent result seems up in the air, but the fact it changed doesn't seem to be.
The Eldar really managed to screw the pooch with that one...