You’re welcome 
Could you maybe print commands to verify that everything looks correct? Could it be that the paths need to be absolute because the current working directory is different when running Processing?
You could also use ProcessBuilder instead of exec which is what I use in the VideoExportLibrary. There I use redirectOutput with ffmpeg and it logs stuff to a file. See https://github.com/hamoid/video_export_processing/blob/master/src/com/hamoid/VideoExport.java#L632