Yes it worked this time, without any error or something.
Now I have another question: Is the process the same under ANY compilation of the Linux kernel? I mean Debian, Fedora, RedHat, whatever. Is it that in general "in Linux OS" first the mount directory must exists, and then do the mounting?

If yes, then in Ubuntu file managers, such as Dolphin, when I mount a volume something like a script ought to create the directory, then execute the mount command, and remove the directory when unmounting, right? And thus there's no way to do the mounting with a nonexistent directory, right?

oops! When I execute:

sudo mount /dev/sda10 /media/tmp_ext4

mount: mount point /media/tmp_ext4 does not exist

No matter what other parameters I use besides (-t, auto, ...), I always get this same message. What should I do?

Yes, I've gained much more insight to GNU/Linux, thanks to 1. so often, your rich supports, 2. my own strong commitment. :-)

YES! That's it man, it worked! I just removed all unity, GNOME shell, lightdm and gdm from tty(1-6) which were my only access to the OS, and installed GNOME shell and lightdm again, and everything is normal again. Something related to lightdm should have been removed the last time I twiddled the packages.

Thanks Matthew.

thank you very much! :-)

thanks marc, it works. Looks like I've gotta study the role of mtab and fstab in a deeper way, since I wasn't using the "-a".
Thanks anyway ;-)