It's a bit of a late reply to the topic but I was looking for a way to use my on/off/on switch for the NVS modes and your post helped!
Still had to do some trial and error, but if anyone needs it:
{down=electric_commands.NVS_MODE_KNOB_EXT, up=electric_commands.NVS_MODE_KNOB_EXT, cockpit_device_id = devices.ELEC_INTERFACE, value_down = 1.0, value_up = 0.0, name = _('PLT NVS MODE Switch - FixedNorm'), category = {_('Left Console'), _('NVS Mode Panel')}},
{down=electric_commands.NVS_MODE_KNOB_EXT, up=electric_commands.NVS_MODE_KNOB_EXT, cockpit_device_id = devices.ELEC_INTERFACE, value_down = -1.0, value_up = 0.0, name = _('PLT NVS MODE Switch - NormOff'), category = {_('Left Console'), _('NVS Mode Panel')}},
"FixedNorm" is mapped to Button1, "NormOff" is mapped to Button2.
If I turn on button 1, it goes to fixed. If I turn off button 1, it goes to norm. If I turn on button 2 it goes to off. If I turn off button 2 it goes to norm. Just like in the sim.