If you're running in main, /dev/mmclbk0p1 is already mounted as /.
And no, don't try to "export" anything from the root partition to /mnt/us (by loop-mounting or so). First, /mnt/us is FAT32 and supports almost none of the important FS features. Second, this is almost certainly going to fail, because /mnt/us becomes inaccessible to the Kindle when it is plugged to the computer as a USB drive.
Just don't do it. Make your changes from the ssh command-line instead.
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