1. Engine malfunctions can happen any time the engine is operating.
2. Engine governor fail to low side means it won't produce much power, or none. Revert to manual mode to restore power.
3. No.
4. As @Jimmy says, as low as your collective is pulled up. RRPM will always decay if the power available is less than the power required. Once you reach that point, the more you pull up on the collective the RRPM will continue to decrease until you reach full up.