You need the serial port connection.
I don't have my paperwhite open, so can't tell from that photo what the parts are.
Yes, I authored the thread, but I used "other people's pictures".
Trash characters will be either a poor ground connection, a mis-matched adapter, or wrong terminal configuration.
You do have a 1.8 volt, low voltage to USB adapter?
Check the ground connection.
settings: 115200 8N1
Keep the battery removed between tests - the battery could be discharge in only hours - depending on how the firmware is mis-behaving.
Let us know when you find out how to put the Kpw into "downloader mode" - -
We do not know the "magic key" combination for the Kpw.