PNP Transistor to switch between Voltage Regulation and Battery Charging
No, when the USB cable is disconnected net VBUS is floating. Its voltage has no defined state. You can add a pull-down on VBUS so it is approximately zero.

However an NPN transistor would be a more suitable choice here.







simulate this circuit &ndash; Schematic created using CircuitLab

This is essentially the same schematic as yours, however the EN-pin switching is being done on the defined VBUS state versus the disconnected Hi-Z one.

The override switch as configured in your schematic allows you to disable the regulator at any time. If you however want to override the USB disable function you are adding, all you need is a SPST switch.  



Aside, with the PNP option, your also burning 500 uA continuously during the operate from battery mode (if you added a suitable pull-down on VBUS, but that may already be there internal to charge IC). In the NPN case, the override consumes no quiescent current when USB is disconnected.