Just a quick note: It finally worked for me!
I just ignored the device for 4 or 5 weeks, supposedly letting the battery drain completely just like suggested above. Then, I held down both volume keys and the power key before inserting a standard USB cable. I let go of the power key after a while and the QUSB__BULK device appeared in the Windows device manager. The flash tool detected the device and flashing a stock firmware image worked on the first try!
Thanks to everyone!