I wouldn't rule that marriage out. I tried to find Mary the younger in 1851 to check out that she was definitely born in Stebbing, but 1851 census records for Stebbing are missing.
Ideally, you would like to see a death record for a William Stock some time between 1916 (or up to 1920 possibly) when Mary was born, and 1841. There are indeed Stebbing entries on the National Burial Index, but I could only find entries for 1802 and 1809 (for a William Stock). So these don't fit.