You can compute different edge detectors by right clicking on a band > filter. I don’t know where the edge index in your study is from so it might be different.
The R2 in my example is the coefficient of determination. It explains the similarity of two raster bands. Looking back, I don’t think it was a good cchoice vcomputing it for the entire raster because this barely highlights their actual differences. Maybe it makes more sense for small selected subsets.
I computed it in ArcMap, SNAP doesn’t feature this. You might be lucky with QGIS though.
Once you calibrate to Beta0 all phase information is lost, but you need it for InSAR.
Preprocessing for PSI is different from the standard DInSAR workflow. Please have a look at the materials collected in this topic: StaMPS - Detailed instructions - #2 by ABraun