Unable to install .apk from /system/priv-app
I think you misunderstand the whole process. You are not supposed to use install command or the likes of it anywhere in the process I mentioned. You simply:


mount the system partition in rw mode
copy-paste the apk into the location, 
set the appropriate permissions using busybox's chmod and chgrp or through inbuilt tools:

adb shell su -c chmod 644 APK    # replace APK with your apk's absolute location, such as /system/priv-app/xyz.apk
adb shell su -c chown root:root APK   # replace APK as said above

do a reboot. 


Follow those instructions religiously. You can altogether avoid the commands using a file manager app such as Solid File Explorer, ES File Explorer, etc. 

During the following boot the Android would detect the apk and install it automatically.