Many many thanks to Jim, my 7C3R, who used his considerable skill and knowledge to finally answer the question of whether the Thomas Sword found in Lovell's Montreal Directory for 1870-1871 could be found on the 1871 Montreal Census at the address noted in the Directory.
Thomas is not one of our ancestors in common, so it really was an act of genealogical kindness to pull together the different resources to sort this out.
The answer, unfortunately, is that the occupant of that address on Census night was not Thomas and family, but was the next occupant, Edward Bell, who is listed at that address in the following year's Lovell's Directory.
Back to the drawing board - but at least that clue was followed up and can be ticked off the to-do list!
Thanks again, Jim!