f. Switching another PC and its attached devices on and off via USB

All newer PCs have a soft on/off switch that starts the computer by connecting the power on digital contact to the ground. A microcontroller needs just a low power low voltage bipolar transistor (100 mA) and it can also short circuit the contacts and start the computer. See the circuit below for details.

But there is another trick! Any PC can control the attached devices by a power relay controller by its power supply. If you want your monitor and printer automatically turned off as your computer shuts down or hibernates, you just have to connect the relay control contact between +12 V= or +5 V= power disk drive supply line and ground. The only additional thing to do is to add a relay coil discharge protection diode that is polarized in reverse. Everything is really simple: