The only reason this diagnostic capability is there is for the Toyota engineering and QC teams that worked on software/hardware. You can read out any variable in RAM in real time, that includes all the diagnostic and operating mode flags in addition to any computed variables. There is other stuff in there too such as dumping out the ROM without removing the chip and grabbing the software version number.
By the way, Vf is connected to a simple logic port of the MCU through a low pass filter. The way it generates the different voltages is by varying the duty cycle of the Vf signal (Port A bit 3 by the way) at the MCU.