AFAICT, you are making changes in the wrong places inside the config files. active-desktop=0 is not supported by slick-greeter so that was never going to work. display-setup-script is only parsed in the [LightDM] and [Seat:*] sections of the config file. You can't just put it anywhere in the files.

Thanks, @DataDrake, those were my thoughts as well, so I've placed display-setup-script in multiple [Seat:*] locations, but none of those config files are read either.

For example, lightdm --show-config tells me display-setup-script=/usr/bin/ldm-session-init is being run from /usr/share/lightdm/lightdm.conf.d/99-ldm.conf so I edited that file to instead run my xrandr command and nothing changed; however, when I ran lightdm --show-config again, it showed my xrandr command being executed.

If you need specific files, just let me know.

Thanks for the help!