Adafruit Gemma unrecognised USB device on Windows 10
I think, you've pretty much done everything. If you could gain access to another board, and try that one you could figure out more. Otherwise your symptoms remind me of a something called an ESD (Electrostatic Discharge) caused malfunction. This basically means one of the system essential chips on the board have been fried. It is very easy to ESD a chip without protection, just walk on a carpet or put on a sweater, and than touch the chip. Chances are you were electrostaticly charged with thousands of volts. The chip does not stand a chance. To prevent this from happening, touch something made out of metal before touching the board.More on ESD: Electrostatic discharge