i and q are the real and imaginary parts of the complex sar information (amplitude+phase).
Regarding question 1 and 2: I suggest reading some basic literature on SAR: http://www2.geog.ucl.ac.uk/~mdisney/teaching/PPRS/PPRS_7/esa_sar_tutorial.pdf
The processing of SLC and GRD products is suggested here: Radiometric & Geometric Correction Workflow - #35 by ABraun