Thank you for the LDS Card Catalogue references to Dewars. I will order some of the films through the Family History Centre in Ottawa.
Vis-à-vis your query about Roderick Dewar born in Scotland about 1817 and married to Mary... The 1861 Census for West Hawkesbury page 188 lines 45 and 46 enumerates a Roderick Dewar born about 1822 in Upper Canada and his wife Mary born about 1832 in Scotland. There was also a much older Roderick (born in the late 1770s in Scotland) and his wife Mary Campbell who were living in Harrington, Argenteuil in the 1840s and 50s. Any connection with the Dewars you are searching for?
Based on information in Donald Keith Dewar's book and on personal communications with a Scottish researcher, many of the early Highland immigrants with the Gaelic name McLure or McLoer adopted the Dewar name when they got to Canada. So you may have more luck finding Dewar ancestors in Scotland if you look for McLures or one of the other early variants of the name. Also, the combination of the first name Roderick with the family name McLure (or Dewar) seems to be localized to the area around the the villages of of Glenelg, Bernera and Galltair (Galder) so this could narrow your search geographically at least.
An index to the old parish records for Glenelg kindly provided to me by my contact in the area, list a Roderick McLoer born in 1818 in Bernera whose father was a carpenter also called Roderick.
Can provide more context for your Roderick and Mary?