Startup Disk full - Read-only- cannot boot Mac Mini
The error your getting when typing in the command means that the command you typed in has invalid arguments. That means that you didn't type the command correctly. 

The reason it's not letting you modify the hard drive is because the drive is only mounted in read only mode.

Mount hard drive for read and write in single user mode

/sbin/fsck -fy 


Then run:

/sbin/mount -uw


If it says something like invalid command try the commands without "/sbin/" at the beginning. 

After your able to mount the disk erasing the files the other website recommended.

Also you mentioned you couldn't boot into recovery mode, but have you tried to boot into Internet Recovery Mode?

How to boot into Internet Recovery Mode 


Hold Option-Command-R or Shift-Option-Command-R on startup until you see a spinning globe
Connect the device to Internet if prompted to do so.


Hope this helps and good luck on recovery your data