I think that is the shorts morph not really sure, and yes I had to stop it at 50%.. it was dragging on for ever and tying things up. Part of the thing I mentioned above on page before here. I don't know nor have found out what was bogging things down yet.
That black artefact is an easy fix with some Photoshop. And when your system begins to slow down try rebooting it. That has helped me when I was still doing renders on my old system.
For dark scenes just light it more so the render works better and darken it in post. A light scene can be darkened but a dark scene isn't that easily adjusted.
For night scenes try lighting it like normal but change the lights colour to blue.
If that works for movies it works for Daz too. This is my darkest scene yet and I also have a blue light shining down on my model.