I did an update and it failed. My KC901V is not powering up (with and without charger). I tried the "black screen issue" procedure without success (backlight looks off so it's maybe not the same problem).
The company appears to be going on holidays from 10 Feb. I don't know if you really meant to mention using KC901S firmware when trying to update a KC901V.
I have had to revert to a previous version on firmware when the update had issues on my unit and blank screened.
1/ Have you pulled the unit apart (carefully) to see the state of charge of the batteries in case one of them has failed? If the series combination is above 7.8V it should be OK to boot up. Otherwise that is where to start but externally charging or replacing a faulty cell.
2/ Did you keep a copy of the previous firmware? I put the old firmware version on the SD card and used the reset but as per instructions and it worked.