You should definitely check Copernicus DEM as well. We did see some discontinuities in Mapzen data, resulting probably from the stitching of various datasets. These could result in “inf” data.
The overlap of the pixels is probably not a consequence of one or another DEM. The actual pixel location is related to the original Sentinel-1 data and the CRS that you are using in your requests. Make sure to always use the original CRS if you want to get the best quality (avoiding reprojection errors).