Apparently "bootmode = factory" allows the kindle to automatically boot to main or fastboot modes depending on which (if any) USB cable is attached. That would be very handy for experienced developers too (not just factory technicians). When I get (or make) a factory cable, I may try that.
FYI, the sense pin is not connected on a normal USB cable, but it is tied low (GND) for an OTG host-mode cable, or tied high (through a pull-up resistor in the cable) for a "factory" cable. It has been reported that connecting the sense pin directly to +5V with no resistor also works (but I would use one, just to be on the safe side). Although USB cables typically contain 4 wires, the microUSB connector has 5 pins, and the sense wire connection is typically made inside the microUSB connector shell.