Don't worry about the password, if you can ssh anyway you have it solved.
Just download a mmcblk0p1 image from here
the one that matches your kernel and firmware. Descompress it and in normal usb mode put it on the main directory, like when using a hack (/mnt/us).
Now, acces the kindle through ssh and follow geekmaster's commands from the debricking post,
dd if=/mnt/us/mmcblk0p1.img of=/dev/mmcblk0p1 bs=4K
Exit the ssh and reboot.
I damaged the filesystem and since I could access the kindle with ssh, I solved this way. I hope it works for you.