I would strongly suggest that there were "avatars" of the various eldar gods before the Fall - well, significantly before the Fall - since it prevents the kind of ad hoc horrendous 'fluff' that forms the body of the so-called "Eldar mythological cycles". They were not the same as the current Avatar, since that was a last ditch 'effort' (or effect) of the fight between Slaanesh and Khorne, but avatars they were. That is to say the 'fluff' stating that gods can manifest themselves fully in the matterium is, for me, horrendous. Rather, they can invest a large proportion of their power into an appropriate avatar...
The position of Ulthanesh (etc.) in this is quite interesting, since the 'level' to which a warp entity can be born is directly related to the 'psyker level' of the individual involved.
Or you can go with the idea of gods physically manifesting. But if you have eldar gods doing that then there is no reason that the other gods/warp entities could not do that. After all, daemons can if but for a short while... Oh, and you would also have to explain why they no longer do it.
As to Ynnaed, then yes I would agree with Rasmus broadly. Ynnaed still slumbs in the Eternal Matrix and has not yet arisen outside of the undifferentiated gestalt consciousnesses of the craftworld entities... And, one other minor little entity. But that's just me... )
Kage