In my case it dowsn't work. I use a Samsung black theme (High_contrast_theme_II) but Poweramp settings is white if I select the Default option (not that I will ever complain about this, the app is just amazing).
I cannot confirm any of these issues. Everything works fine on my Smasung A3 2017 with latest original ROM and HighRes output.
EDIT: WRT point 4: I managed to crash the app pressing the "Back" capacity button whilst in the Queue page.
I tried it, after a reboot I can select the Hi-Res audio but as soon as I play a track the app prints "Failed: Poweramp Hi-Res Exp Output. Using default output."
My opo (6.0.1) doesn't have the line you mentioned in the build.prop file but only the 16bit one. Does this mean that it is "optimized" out as you mentioned? Would a local.prop identical line help?