Stitching multiple SLC frames is not possible with ISCE. Each frame is typically focussed with different parameters and they cannot be seamlessly stitched into a larger frame.
The SAOCOM metadata does not contain information about the angle of incidence, however the xemt file provides the acquisition beam. This file is downloaded separately, so its path needs to be configured. The xml is usually located in the same folder as the image, but since they are separate files, as the xemt, file location is required.