You completely think wrong if you think the mint repo system could somehow be a mirror of the debian repo system.
Your last question i already answered in your apt-pinning thread. Except the function of the backport repo. Backport
mostly is empty, only in use sometimes in order to test mint specific developments.
I believe in everything, except humans.
LM9, Debian Testing Xfce, Debian Sid Xfce - CPU/Amd64 2,6Ghz, Graphics/Nvidia 9600 GT, Sound/SB Audigy4 emu10K1