What a great description. Thanks!
I admit, most of this made sense, but you've done a great job of explaining the rest. So, if Vin is less that 2 X VCC30, then USBVCC is connected to +5V? Otherwise, the power is supplied from the on-board regulator, from Vin. And VCC30 is generated by the USB chip? It makes sense now, but was not obvious to me. Thanks!