Alright, this messed-up Windows install is annoying me too much to continue. Time to re-install Windows 10 again to see if it'll actually work properly this time...
Edit: I couldn't find this info anywhere, although it tells you after you click the Delete everything option.
Reset PC ->
Remove everything option also wipe/format any other installed drives and partitions?
The answer is: it's up to you.
Windows will tell you that there's more than one drive installed and will ask you if you want to just format the drive Windows is installed on or format all drives.
Now you know.
Edit 2: Damn Windows installs fast. 6-8 minutes, maybe?