I used the first on on your list.
I used dfu-programmer to flash the hex file and it looked as though it worked because the com port was there in the IDE. I've tried to put the mega 2560 back into DFU mode by shorting the two pints closest to the USB port but nothing changes,
Do you or anyone else know of a different way to put the Arduino Mega 2560 into DFU mode?
If I cannot get the USB working is it easy enough to program the Mega with the serial port?