Clear large .MobileBackups files/folder?
All signs point to you not having 400 GB (or even 26 GB of space used from .MobileBackups) and with the repeated edits, I'm not even sure we're on the same page, but for others - here are some things to try:

Rather than re-enabling things, try giving the system 5 minutes to clean up before the restart

sudo tmutil disablelocal 
sudo shutdown -r +5


Then when the machine restarts, look for that folder and delete things using Finder or terminal from /.MobileBackups once the machine is restarted with the local store disabled.

First check for a huge file in / with:

sudo ls -sa1 / | sort -n | tail -4


We know the large files aren't anywhere else but Applications and Users - but you could use this answer to search for smaller files (say 1 GB or larger)

If you're looking to just know what the size of various folders is - this long running command (14 minutes on an SSD MacBook Pro with 200 GB total allocation / 6 minutes on Mac Pro with about the same) will show you a list of the largest folders - hidden or not.

sudo du -smx /* /.* | sort -n


If that folder no longer shows using that space, your next recourse is to see if the filesystem catalog and free space accounting can be fixed. That would be the last step before backing up, erasing and reinstalling the OS.

Boot to single user mode: Hold Command + S when the boot chime is sounded and hold it until white text on a black screen appears.

/sbin/fsck -fy /


repeat the above check+repair until there are no errors and then 

halt