I have another LAING mystery, can anyone help solve it?

1851 census Crichton - Midlothian, Duncan Laing, Schoolteacher, aged 50 from Torryburn in Fife is living with his wife Mary Ann Howell also 50 from England. With them is Mary Ann Dewar 9 and it says granddaughter.
I do have a son for Duncan & Mary Ann also Duncan born 1824 in Dunfermline Fife and living with them in Crichton on the 1841 census, he is a Joiner. But if Mary Ann Dewar was his, her name would be laing. So I started looking for a daughter but no trace.
Now in 1861 Mary Ann dewar, teacher, aged 19 is still with the 'Grandparents' but done as a Lodger this time not as a granddaughter. By 1871 she is missing so i found a mariage in 1863 of a Mary Ann Dewar, Teacher to a Robert Robson but her parents as down as John Dewar and Ann Green.
I cannot trace the son Duncan after the 1841 census nor find anyother children for Duncan & Mary Ann in Fife or Midlothian.
Anyone come across these names? Thanks Evelina
I have just re-looked at the mc and the couple on the next wedding the Mothers name is Anna Bruce nee Laing. Now that must be a coincidence or did the registrar in 1863 mix up the mothers maiden names?


??/ Any one come across anything like that?
Mary Ann Dewar's dc does have the Ann Green as Mother but her husband registered the death and he may have copied the names from the mc.