I'm not entirely sure why you are looking for a death certificate for Thomas or Joseph ROWELL. I appreciate that Mary Ann has said this is the father's name on Emily Adelaide's birth registration, however he doesn't appear on her baptism record (so far as I can tell - I haven't seen the image, just a transcription which only has mother as Mary Ann CHARLTON). Mary Ann says she is "unmarried" in 1871 not "widowed". The Joseph ROWELL, commercial traveller could be just a name to make the birth seem legitimate, or he could be a real person, but unfortunately we have no other details about him at this stage. He is not with Mary Ann in either 1861 or 1871, and I can't see any Joseph ROWELL, commercial traveller anywhere in either 1861 or 1871 anywhere in England. Of course he may have changed occupations.
Emily Adelaide did not know who her father was when she was married, which adds to my suspicion that her parents were certainly never married. Do you know if George Robert married? - I wonder who did he say his father was?