Each audio output channel is only one signal.
1) You can't. This is a mono output.
2) No. You have to write your program to determine what audio is sent out what channel. You can mix stereo audio source into mono so that audio and left and right are combined to a mono output. I don't think this is what you want to do though.
3) No, you will only have 2 channels with 2 speakers on each channel.
If you want 4 channels you can use two of the Analog outputs in addition to the audio outputs. The audio quality won't be as good on the Analog Outs, but, for example, if you are driving a subwoofer or mid-range speakers with those channels then the woofer speaker's low-pass response will filter out the high frequency digital noise from the DAC and it will probably sound really good.