EV3 not recognized by PC when attempting firmware update after interrupted update
You have done the right thing by contacting Customer Service. They'll be able to help you potentially fix this or replace the part if needed.

I know you said that you already tried this, but I wanted to list the steps for a firmware update here in case others find this question. Here's the exact language from the EV3 user guide:


  Forced Firmware Update
  
  If  you  need  to  update  your  firmware  manually,  follow  the 
  steps   below.  (Note: the EV3 Brick still requires a USB connection
  to your  computer) 
  
  
  Make sure your EV3 Brick is turned off.
  Hold down the Back, Center, and Right buttons on the EV3 Brick.
  When the screen says “Updating”, release the buttons.
  
  
  If the manual firmware update does not make your EV3 Brick work  the
  first time, repeat the manual update process.


Here are some general things to try if you haven't already:


Take out the EV3 battery and unplug from your computer and just let the brick sit for 30 seconds or so. Repeat the above steps.
Reboot your computer, or try using another computer, and repeat the above steps.
Try using another USB cable and/or USB port and repeat the above steps. This is unlikely to help in your case since the cable just worked for you, but it could be helpful to someone else.


Please post back here if Customer Service is able to provide a fix for this issue, as I'm sure it will help out someone else.