In terms of your first question, the backscatter signal between snow free and snowy areas depends on the terrain of your AOI. If there is terrain in snow free areas then the backscatter will be much higher compared to areas with wet snow. If your AOI is relatively flat, then the intensity of the image acquired over wet snow between the the image acquired overt flat area can be very similar and hard to discriminate.
I do not really know what methodology you have followed to extract wet snow. I can give you my advice on that (which might not be the most appropriate in your case, you need to do bit more research on your own)
- first of all you need to determine the backscatter of the snow free area collecting a time series of data.
- Collect data over the snow area.
- You might need to coregister the SAR data collected
- Use the ratio between the image acquired over snow and the image acquired over snow-free area
5 determine an appropriate threshold
This is roughly the methodology i would follow.