How credible Ynnead is debatable, as is how old Slaanesh is as a being. It's implied in places that Slaanesh existed way before she was ever 'born' and may of engineered her own birth. The Eldar are not all seeing, nor all knowing, it's why the screaming cage was such a big deal...they don't actually know that much about how Slaanesh works and the tome they created needed to be turned in to the Black Library so the Harlaquins could reassess their situation. Ynnead is merely a sparkle of hope, but Divination is neither perfect or accurate. It wasn't entirely right about Argal Tal, and even the Fateweaver can be outsmarted and undermined.
There is no time in the warp. That is why there is reference to Slaanesh being new, but having always been there.
Like everything else in the warp: Yes and no. There is time, but it fluctuates, it goes backwards on some planets, while somehow going sideways on others, time is a concept that exists there, but it changes rapidly from place to place. Two planets might be so close you can touch them, but your hand passing to anothers surface may take a thousand years.
We need to kill the Space Marines, then kill eachother. Then with our disembodied souls we'll possess their corpses, return to Terra, and conquer it in a Necromantic Heresy!
First, we will dance the Orks into oblivion. Their moves are weak and they lack flair. It will be child's play. Next, we will be overrun and consumed by Tyranids because we ignored them. At least they can dance.
Ey find yer lack ov Dakka disturbin'... *sudden realization* HOO YOUZ KALLIN' WEAK YA RUNTY TWINKLE TOED GROT LOVER!?
