Until you enable SSH over WiFi in the USBNet settings, you should *NOT* have any issue with passwords, dropbear is patched to let you in no matter what (not OpenSSH, but dropbear, the default, yes).
That said, to get in over WiFi, you'll need the Kindle IP address for the wireless network interface. It should be in you usual subnet, since it's connecting to your usual router. It should be visible on the ;711 page, IIRC, or you can simply run ifconfig over USB

.
Not sure if ICMP pings are blocked by the firewall over WIFi, so the fact that it doesn't ping might not be a good indicator.