I have both Guthrie/Spence in my line and I did come across a tree having James Maxwell Spence m Margaret Guthrie b 1805 daughter of David Guthrie and Margaret Williamson. I cannot attest to its accuracy but note the author has her death in Australia in 1897 and perhaps this is where her parents were listed. I also checked some the family trees with the Orkney Family History Society which basically confirm the above but no mention of this Guthrie's siblings.
I have the following children in your line, but no Christopher. The only Christopher I have b 1820 belonged to William Spence/Katherine Harvey so I would like to know where you found yours.
13/09/1816 SPENCE MARY WM. SPENCE/MARY GUTHRIE
18/12/1818 SPENCE WILLIAM WM. SPENCE/MARGT. GUTHRIE
10/11/1821 SPENCE EUPHEMIA WM. SPENCE/MARY GUTHRIE
12/03/1823 SPENCE ARCHD WM. SPENCE/MARY GUTHRIE
11/06/1825 SPENCE JO WM. SPENCE/MARY GUTHRIE
Not sure if you have Mary Ann's 1815 marriage bann, but the witnesses were Helen Spence and Mary Guthrie. This Mary could be her mother or possibly a cousin which might offer you more clues.
And, finally the only 3 Mary's on record as being born between 1760-1800.
16/01/1790 GUTHERIE MARY WILLIAM GUTHERIE/ISABEL MOWAT KIRKWALL /ORKNEY 021/00 0030 0070
11/11/1794 GUTHERIE MARY DAVID GUTHERIE/JEAN MICHAL SHAPINSAY/ORKNEY 028/00 0010 0165
20/11/1773 GUTHRIE MARY EDWARD GUTHRIE/ELISA. FOTHERINGHAM SANDAY (LADY CROSS AND BURNESS)/ORKNEY 026/00 0020 0052
HTH
Don