4. What is the function of the M15's HOLD switch?

The HOLD switch can effectively prevent misoperations. When the HOLD switch is set to ON, all buttons (except the Power button) will be disabled by default. But you can set which buttons to be disabled in Settings -> General -> HOLD switch setting.

Note that when the HOLD switch is set to ON, the volume cannot be turned up, so as to prevent excessive output volume. If the volume knob is turned to a higher-level position during HOLD and you wish to adjust the volume after HOLD is off, you'll need to first turn the knob to the minimum level before adjusting it again.