You can leave the gateway blank. No internet access from the kindle (i.e. 3G tethering), so no gateway needed on that USBnet interface.
Ping works if USBnet up. SSH connection rejected fails if dropbear down. From a main boot, you can do "mount /dev/mmcblk0p2 /mnt/mmc" to explore diags partition to see what is in /mnt/mmc/usr/local (where dropbear files should be).
Or you can use ixtab's dropbear installer...
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