edit nvidia.conf and change the screen section to look like this. that should fix most/all of the scaling problems
Option "UseEdidDpi" "False"
Option "DPI" "96x96"
so you now have a fully, properly booting system? no more manually starting x?
ah yes, the wonderful world of kde compostitor . i too get graphical glitches after wake where my latte-dock looks like its made of high definition static (at least its hd ). that nvidia.conf fix i listed above may improve that. other than that you can try different compositor options and see if it helps.
you can do that, it likely would not make a difference for the compositor glitches. that could be fixed by having the compositor restarted after suspend which is easy enough to do with a systemd service, i think i have one laying around i could easily modify to do that.
since you know where you went wrong with the xsetup, backing up your current setup before returning to the old one and doing prime setup over would not take very long, the tutorial only took you what maybe 10-15 min?
the rest of the time was spent troubleshooting human error, which has been the case with anyone who follows that tutorial and runs into issues, including myself. now that you know what to be careful of you could do that easily.