Mac – Will Time Machine backup an iTunes library that’s stored on an external HD

This probably can be achieved by just using iTunes Match, don't know if you're willing to subscribe to this. The master library with AL is getting matched, giving you access to the AAC versions from the cloud. From then on, everything you do to a file in your AL library is getting automatically synced to the iCloud AAC version.

When you're on the go just switch to the local SSD library. Start with an empty library, activate iTunes Match and all your music get's visible without being physically on the device. You could then download all the AAC versions from iTunes or just those you want to have on the device. All changes between your master and this slave library should get synced, you just have to manually download music on your SSD when you add something new to your master library.

Time Machine allows you to backup multiple macs to one drive. You just plug it in, set it up, and you're good. If you look at the file system, you can see separate backups for both computers. At our home we currently do this with a Mac Mini and a MacBook. The Mac Mini is always plugged in, and the MacBook gets updated every couple weeks. We use a 1.5 TB drive from Western Digital. Given your description, I think this solution with your 2TB drive should work well for you.

The data backed up by both macs will not be the same, as no matter how hard you try, both computers will wind up having subtile differences, so the data won't be "exactly" the same. Backing up only one system doesn't seem ideal to me, but you can restore a MacBook from a Mac Mini backup. It works fine.

Also, you realize that the drive must be connected for you to access the photos and music that you have stored there. You'll have to factor that into your storage solution.