I'd just love to see @cxg2014 come back and confirm all the steps he took, for other people having the same issue on the same device.
I have disassembled the 'update_kindle_8th_5.8.7.0.1', and I am suprised to see that they do in fact ship with "U-Boot 2014.04 (Jan 16 2017 - 03:45:34)", while the PW3 update 'update_kindle_all_new_paperwhite_5.8.9.2.bin' still has "U-Boot 2009.08-lab126 (May 04 2017 - 10:02:34)"
I now realize that they are newer devices, I just assumed that they used the same basic platfom as the PW3. But I clearly have not been paying enough attention.
In general, this could be a good thing, making some steps of recovery easier, if they start shipping this on more devices that have serial ports ( as far as wiring up a serial port is considered "easy"
)
At any rate, congratulations on getting your KT3 up and working again @cxg2014