WallaceWalker on Family Tree Circles

Looking for more information on the ancestors of my wifes ggrandfather Robert WALKER born either Stevenston Ayrshire or Greenock Renfrewshire Scotland , but according to IGI was christened in 1830 and 1831 in Greenock , his parents were John WALKER stonemason and Mary Ann BOWEN ,siblings were Catherine born 1825 Portabello Midlothian and married Adam KIRKPATRICK circa 1848 and later migrated to Victoria Australia , Ann born circa 1828 Portabello and married James MCPHAIL and later migrated to Canada , Mary Ann married Alexander MCLEOD circa 1852 and died Greenock circa 1868 , cannot find any trace of the mother Mary Ann on the 1851 census or sibling Joan ( transcribed Ivan from 1841 census )
Robert came to Victoria Australia circa 1848 according to his death certificate maybe on a ship called the Ann Milne and during a period from about 1862 till about 1876 called himself Robert WALLACE and even when he reverted back to WALKER named nearly every child with the second christian name of WALLACE , guess my mail line of enquires are , Why did he use the name WALLACE , what were the origins of his parents , what happened to them ( i believe John the father was deceased by 1868 )and what happened to Joan ???

Please someone unlock this mystery !!!