I am thinking whether it is necessary to have schottky diode for protection. I think normal diode is good enough. The regular diode turns full on about 1V. The point of using a diode is to prevent reverse bias the base emitter diodes of the output transistor. Base emitter diode of transistors behaves like a ZENER diode when reverse biased, it will break down about 3 to 6V. Once the base emitter diode start to conduct, you risk burning the transistor. But the base emitter diode does not conduct until you have at least 3V or so, a normal diode will clamp at 1.2V maximum, so this will protect the transistors already.