I don't follow your argument. The person selling the bit coin in 2 assumes that when the number in their account balance goes up by USD_FAKE that there was real

I don't follow your argument. The person selling the bit coin in 2 assumes that when the number in their account balance goes up by USD_FAKE that there was really money to back it up. They have no way of knowing that Mt Gox didn't ever have that money. I think that is the point they are trying to make.