referring to points 1 and 5: Please specify. “it doesn’t work” is not helping us to understand what might be the problem. Please post error messages, possible results and why you think they are not correct.
to point 2:
SNAP uses the quantifaction value from the metadata to convert the raw data to decimal values. This allows to store the raster data as integers (and saves lots of download time and disk space). You can read about it here QUANTIFICATION_VALUE of Sentinel-1C and here Radiometric conversion of sentinel 2 images
to point 3:
maybe the literature values refer to spectrometer measuremens (contact probe). Values from satellites depend on the spatial and radiometric resolution of the sensor, as well as on proper calibration. Have you checked if L2A data is available in your study area?