I think, I need to describe processing stages in LibRaw (simplified case, bayer image)

1) open_file() - reads metadata (EXIF and makernotes)
2) unpack() - decodes file contents into imgdata.rawdata.raw_image.
COLOR() call is useful after that: to know what color has pixel at (row,col).

3) dcraw_process():
- do raw2image() internally, allocate imgdata.image[] and populate
imgdata.image[row*width+col[COLOR(row,col)] = rawdata.raw_image[(row+top)*raw_width+col+left]
- do white balance
- than bayer interpolation
- and other possible postprocessing such as denoise or highlight recovery
- than output color conversion and data scale

After that, image[row*width+col] has [0..2] components filled with RGB values and something in [3]

4) dcraw_make_mem_image() may be used to create 3-component bitmap (with gamma correction), in 8- or 16-bit per component to be written into TIFF/JPEG or displayed on screen..

That's all that simple :)

You may repeat steps 3 and 4 with different imgdata.params settings to get different renderings