8bit vs 10bit vs 12bit
I can answer your first question. At the same bitrate and transcoding from an 8-bit source 10-bit HEVC is better than 8-bit HEVC, because it hardcodes dither. https://gist.github.com/l4n9th4n9/4459997
I am still unsure, whether that makes 12-bit HEVC transcodes superior  to 10-bit, thats why I ended up here.