There are a lot of questions there, but the answer is quite simple: the disk image itself is just a file like any other. The fact that it represents a disk image, or that its data is encrypted, doesn't really matter.
When you back up your user files, the disk image will be backed up as well. So, there's no need to do anything special -- I'd just endeavor to ensure that the image is unmounted and closed when you do your main backup.
If you were to try to back it up as a disk "drive", which you can do if you mount it and use it as a source, the result won't be encrpyted, which basically misses the point, so don't do that.
Make sense?
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--Dave Nanian
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