Hello Matsumoto-san,
1. This diode is used to increase the discharge voltage of the inductor coil. The zener reverse breakdown voltage can be whatever you choose. Around 5V or 12V is appropriate.
2. Yes, 500mA is the recommended maximum current for a given channel. Other limitations to consider are that the device cannot handle >2A at a given point in time.
Additionally, with multiple channels running, depending on the duty cycle there is a recommended maximum current based on thermal limitations of the device. This is described in Thermal Characteristics, section 6.8.
Best,
Michael