The central question is are they technically still the same race?
Yes they are the same race.
- If they are can a Dark Eldar to become an Eldar ? i.e: Had enough of Hedonism, debauchery and join a craftworld?
Dark Eldar can join the Craftworlders but it’s not easy, it’s a lot easier to fall then it is to get back up. Normally if they have grown tired of their former lives they’ll leave and join a corsairs group as its much easier than going cold turkey.
- Can anl Eldar become a Dark Eldar? Decide torture and similar 'kinks' are fun and join the DE in their webway city (as an actual member and not as an amusing torture victim)?
Yes but they normally become rangers first then if that’s not enough excitement for them they’ll join the Corsairs before finally taking the final plunge into depravity by becoming one of the Dark Eldar.
- Can Dark Eldar and Eldar breed with each other? if so what happens to the child?
Depends on the parents really but I would probably say that they would be bought up on one of the Craftworlds.
- How much do other factions know of the difference between Dark Eldar and Eldar?
Most don’t bother to find out; one filthy xeno is the same as any other. Surprisingly the traitor forces within the eye seem to have a better grasp on the differences between the two then those outside.
- If a Dark Eldar force raids an Imperium / Tau world do the races blame the Eldar as well?
Simple answer to this is yes, yes they would.