As soon as one has accessed /proc/mounts, /proc can't be umount:

# settrans -ga /proc
settrans: /proc: Device or resource busy

This is because mtab is still mounted on /proc/mounts:

# settrans -ga /proc/mounts
# settrans -ga /proc

settrans -R is supposed to be doing this, but unfortunately this is not implemented yet in libnetfs, see #ifdef NOTYET in netfs_shutdown.