Nigel, WP, thanks a lot!!
How about if external regulated 5v is connected to the 5v pin of the Arduino uno, will again it be possible to connect the PC to the Arduino uno through the USB port?
I know that it is not recommended to connect external 5V to the Arduino uno 5V pin, but it was done many times successfully. As long as the external 5V is stable, there's no problem to feed it to the 5V pin of the Arduino uno.
By the schematics,
It seems that the gate of T1 would be LOW, while both sides of the T1 will be 5V.