Hidden current loop on power supply
Instead of cobbling together discrete parts and hoping to get the result you are after, this is more simply and foolishness-resistantly done with a dedicated IC: Linear Tech's LTC4412.  The circuit below should get you started.







simulate this circuit &ndash; Schematic created using CircuitLab

Note that PWR_CTRL in this circuit is HIGH to select Vbus and LOW to select Vusb when both supplies are connected.