With respect to the syncing 3-12GB every time you open Zune, perhaps what's happening is that when you open up Zune, it's trying to update your tags and album artwork, or change the "last played" metadata in the file. The changed tags cause the entire file to show up as different from one machine to another, so Live Mesh jumps in and tries to update the files on the other machine.
As a general Zune recommendation as well as your specific problem, I would disable the "update track information and album art" option in the Zune software on both machines, and fix any tag problems you have in a 3rd party editor such as mp3tag (it's free, and faster than using Zune anyway). Both the automatic tag update feature and manually fixing tags in the software are notoriously buggy, and once I started using mp3tag it solved all my library problems.
Hope this helps.
