Yes that would have been what is now the Old Montréal (Vieux Montréal).I would think that at the time the entire city did not exceed one square mile.
Your wish: going back one generation beyond William Patton and Elisabeth Greig; is this correct?
Who was born in Québec?Approx. year?
If William's father was not his witness at his marriage, this may be an indication that his parents did not come to Canada.And if they came, they could have been married in outside Canada.
Did you see the marriage record? Were William's parents' names mentioned. The Mormons (LDS) have microfilmed the recordsof Beauharnois and Chateauguay Presbyterian Church, 1834 to 1899.Microfilm 2311 according to our collection.
Birth/baptism records are one way of identifying grand parents of a child or uncles and aunts.I have reconstituted families with this approach.