Incidentally, you are correct that there are errors in Hew Morrison's copy of the Register published in 1911. Robert and Janet's daughter Janet was baptised on 30th October 1803 (missed from HM), and the child baptised on 30th November 1804 was indeed William.
A sponsor appears to have been the person who presented the child for baptism if a parent was not present, so I presume Robert was absent and the record should be read as "Janet, the wife of Robert, presented the child as sponsor"