Re: Darla In Heaven: Darla appeared in "Inside Out" as a ghost in a white outfit, claiming to have been sent by the Powers That Be. So, Darla survived death, and is on good terms with the Powers. and is dressed the same way as Cordie did when when she was in heaven, and the same way as Joyce did when she spoke to Dawn. Plain reading of the text, she's in heaven. Or, one of the near infinite number of demon dimension controlled by the Powers that be that we conventionally think of as heaven. I believe both Skip, Illyria and Tara have confirmed that heavenly dimensions are a thing, as did the gang when they stole the Delphic Tripod.
I would contend my position is sound based on the available evidence, but, season four is a little crazy, with all the Jasmine's conspiracy, and the regular ghosts and the First Evil, and a lot of unanswered questions as to who was whom, so I am willing to accept that other good hypotheses exist on this topic. Yes, Darla could have been the First trying to convince Connor to keep Jasmine from being born. Yes, the same could be said for Joyce or Lilah. Yes, Buffy's soul could have wound up in a random demon dimension, or heaven works more like Elysium or Valhalla where only the truly worthy go. Yes, the Powers could just be inhuman higher demons with foreign concepts of morality who have decided that that humanity is worthy of something other than cattle or slavery as opposed to divine or angelic. There's no series bible, there's no answer and there's not going to be an answer, so it's YMMV.