a) The osc application has some boxes you can select (don't remember exactly)- Michael will help you.
b) These are the raw signals (same module/file where you are using the channel data)
wire [13:0] adc_data_a_s;
wire [13:0] adc_data_b_s;
c) It is how you set it to be (default is 2's complement I guess) -
You can extend it like this -
{adc_data_a_s[13], adc_data_a_s[13], adc_data_a_s};