There is no good reason why any Eldar would worship or be on Slaanesh's side. There is no benefit for an Eldar. I suppose one could, but that Eldar would be probably the stupidest creature in the galaxy. Dark Eldar don't even worship Slaanesh and they are straight up crazy. (But I love them.)
Eldar worshiping Slaanesh although possible it is something every Craftworld Eldar will learn about from the earliest age, Every Craftworlder is taught how to shield their mind to give as little as possible to attract she how thirsts. As for possession again not impossible but any daemon that was to try and us it to their advantage would be in for a horrid surprise, All Eldar are psychic this includes the Dark Eldar (they psychic ability is just shrivelled up and dry). In "Path of the Warrior" there is a former Dark Eldar who is a scorpion and living on the craftworld he is able to interact with all the tech the craftworlders have including the aspect armour, the seers and healers of the craftworld put him through some extensive healing. In "Path of the Seer" the main character tries to pull a daemon out of a possessed human to try and save them, the very fact she tried to do this is incredible, as it implies that for Eldar Seers this is possible, otherwise she would not of tried to do it. So possessed Eldar will be very bad for a daemon as it will be detected and destroyed, not cast back into the warp.
It's not really the same, but the closest any Eldar (that survives for more than a millisecond) gets to 'worshiping' Slaanesh is probably the Solitaires, who personify Slaanesh during Harlequin Masques. They have be made Nulls by Cergorach to do it, and even then have something like a ~50% chance of Slaanesh claiming their souls when they die.
They do take on the role of Slaanesh, and I would say that perhaps this is why they are regarded as damned; they aren't nulls as such; they still have psychic senses and so on, but they are definitely changed in some way. Iin fact, the Solitaire's sacrifice is in some way said to protect the rest of the Masque. Perhaps it is the fact that their soul is forfeit to Slaanesh unless Cegorach can trick the Dark Prince out of it? But as you say, they do not worship the Young Prince as such.
Not to nitpick - but Slaanesh is variously the Dark Prince or the Perfect Prince, while you may be confusing that with the ceremony to embody the Avatar of Khaine: The Ritual of the Young King. In this ritual - the chosen exarch embodies Eldanesh as he stands against Khaine - so the Young King in this reference is Eldanesh, not Slaanesh Eldanesh eventually loses to Khaine, but in so doing Khaine's hand is forever bloodied. The bloodied hand is in some ways a mark of his savagery - but also an acknowledgment of Khaine's mortality (he has blood, and he can bleed). For the ritual to be relevant to the embodiment of Khaine by an exarch, it supports the suspicion that when Khaine slew Eldanesh, Eldanesh embodied Khaine: his death tethered a god to himself - such is how a god bore a mortal wound. It also stayed Khaine's hand - prior to which he was set upon genociding the Eldar race from existence - after the bond - he could do no such thing as we now bore part of his soul. The Ritual of the Young King then commemorates this bond - that deep within the Eldar - those who commit themselves fully and completely to the Path of War without recourse, without doubt - are able to draw upon this connection - not to sacrifice themselves upon Khaine's altar - but to embody themselves with his eternal spirit. Further supporting this is the reference to Eldanesh as a King, for Kings are a very specific form of political power manifested through Divine Right. Eldanesh had many names, but to reference him in the ritual as a King - is to further allude to his bond to a god: Khaine. Edit: How did I only just catch that Eldanesh and Slaanesh are such similar names? And that both are referred to as Kings/Princes - both references to the same otherwise unused system of government amongst the Eldar (monarchy). It may also be interesting to note that while Eldanesh was marked as a King - Slaanesh is only a Prince: implying his path to power is incomplete. This may support my past mention of Ynnead being a rebirthing of Slaanesh: reborn with our newfound discipline and maturity, rather than manifested only of our old excesses.
Actually Slaanesh, being the youngest of the Chaos Gods, is also known as the Young Prince. I know about the Young King EDIT: You've got some interesting interpretations there
That´s actually a really difficult question. I guess averaging 1000+ years in general works, but it really depends on the individual Eldar, the path/-s he/she walks and your definition of lifespan. The stronger they develop their psychic powers, the longer they life. However, Farseers for example, if not killed, start to crystalize sooner or later and are directly absorbed back into the Infinity Circuit so the actual time of "bodily death" varies greatly. For Phoenix Lords and Exarchs, being technically immortal, it´s more of a philosophical question, as the armour contains the original consciousness, as well as the spirits of all Eldar who wear the armour later. Once an Eldar dons the Exarch armour, he "fuses" with all the spirits before him, takes the name of the original Exarch and is the new/old Exarch. If one puts on a Phoenix Lords armour, the Lords consciousness takes over the body and the Eldars soul just "dissolves in the nothingness of the armour and is gone" which in my opinion is the only way any Eldar gets to have an eternal, peaceful death. So... does the Eldar die once he dons an armour? Does he die with his original body? Is every Eldar who becomes Exarch immortal because he always has a body and always has a "soul" (or lots of them) or is the first Exarch the only immortal because his name defines all that follow? Or is no Exarch immortal?
There is a hypothesis that links the Eldar lifespan to the Webway. I'm not sure how true it may be, but Inquisitor Czevak, after spending some time with the Eldar and having a really good time there, emerged from it younger and stronger than himself getting in there (he was old and wrinkly and scrawny).
