The Picture and Sound buttons don't send commands. They are shortcuts to a customized Device mode of the devices being used for audio and video in the current activity. So, for example, if you are in the "Watch DVD" activity, you could press the "Sound" button and the remote would be dedicated to the receiver device, where you could adjust surround modes, speaker levels etc.. Press "Sound" again to return to the "Watch DVD" activity.
You customize the "Picture" and "Sound" modes from the "Customize buttons" page for either the device or the activity. Select "Picture" or "Sound" from the drop-down at the top of the page (it will normally be on "Activity" or "Device" depending on the page you are on).
Yes. Just select "Settings" for the appropriate device under the "Device" tab, then "Customize buttons". You can re-order by typing commands in the order you want (and selecting the appropriate command from the drop-down box), and delete by leaving the label column blank.