This scheme, for example, could not be used to switch 12V like this
To which "this" are you referring to? And, if you don't mind, I'm curious to understand why it cannot be used to switch 12V like you say.
Well, dang. I've actually already prototyped a board with the source and drain switched (my test circuit showed it to be correct, which I now see the flaw). So given my test board, I'm trying to predict what is going to happen if VIN is powered and the USB is plugged in. I assume it is not desirable if there is a potential difference which allows current to run backwards into my laptop through the +5V line. I just don't know if that will damage my usb ports or if it will be ok. The +5V output from the converter will most certainly be ~5V maybe slightly less (4.95). My converter is a buck converter so inrush on the output shouldn't be an issue because of the inductor (AFAIK). So I assume that as long as the USB +5V > than the converter +5V output then the potential difference will keep the current from flowing back into my laptop. Inevitably the current will flow back up so I'd like to know what to expect. Do you know if this will damage the usb port?