I've searched the Lovell directories for Montreal, for a number of different years. I believe I found my John McMullen in 1848 and 1849 as a laborer, and on the 1850 directory as a shoemaker. However, I have not found an 1851 directory for Montreal, as it seems to jump from 1850 to 1852.
I'm wondering if someone is aware of a searchable directory for 1851 for Montreal?
I've been unable to determine John's fate by 1852. His wife shows as Margaret McMullen in Montreal on the 1852 census, with her 2 boys. They are with a Johnston family. Margaret still shows as married, but John is not on the census.