I booted up Mint LMDE on my spare PC to do some tests.
I have discovered there is no user "root" so you cannot login as root.
Using sudo or sudo su only gives you temporary root privileges as you have observed.
Mint LMDE is neither Ubuntu nor Debian and is basically a constantly evolving Beta, and unfortunately the
lead developer has suffered some personal setbacks and can no longer continue to support the distro.
See this thread for details.http://forums.linuxmint.com/viewtopic.php?f=58&t=61134
With that in mind, my advice to you is to use a well tested and trusted distro
such as Mint 10, or Debian Lenny or Ubuntu.
Mint 10 has a "root" user account built in and you can login and run as root if you so choose.
I know you are aware of the risks.
Things are more like they are now than they ever were before.
Dwight D. Eisenhower (1890 - 1969)