I think I gave you enough as i can , i gave you pictures and what equipment I use
But the diode only protects the transistor , of how many negative volts? the diode is only 0.5 volts, so is the negative voltage is more than -1 volts it will damage the transistor because the protection diode is only 0.5 volts to turn it onTherefore the diode is added so that the base does not get a negative voltage that would damage it.
When do you need to add hysteresis to a transistor switch? when the input on the base is what kind of signal?Normally a transistor switch has no hysteresis. It simply switches on or switches off.
You can add hysteresis to a transistor switch if you want.
Why would positive feedback give you hysteresis?Hysteresis is simply some positive feedback