yes, you well understand my question.
i think the problem is the control of mosfet T1, it depend on state of VIN, that is 0V when the board is powered from +5V pin directly.
i read the thread you link, so there is also another problem due to voltage regulator that in this condition, is reverse powered.
i think that with 2 little modification on the board the 2 problem can be solved (i will test ASAP).
1- a diode with anode to +5V and catode to VIN to protect the voltage regulator IC1 from reverse powering.
2- remove resistor RN5D so also with 4,4V ( 5V-Vak of new diode) can trigger the comparator IC7B
modify the board in this way, let all functionality as before, it can be powered either from usb, X1 connector, and +5V, but when the last 2 method are used, usb power will remain isolated from board.
what do you think ?