More on the newspaper report of Mr and Mrs Caldwell, after marrying, sailing for England in June 1900 on the Oceanic of the Dominion line. I can't find that sailing, but as you say, if William Caldwell was crew, they wouldn't be on a passenger list. They would be on a crew list though? Or one crew and one passenger.
According to the Ships List website, there were two different Oceanic ships. One built 1870 for the White Star Line and scrapped 1896 in Belfast, and one built 1899 for the White Star Line, with the maiden voyage 6/9/1899 Liverpool to Queenstown (now Cobh), Ireland and Boston, continuing with this route until 1907. The Ships List also has info on the different sales, ownerships and combination of lines in this era, and it seems that the Dominion Line name continued to be used, but their fleet list doesn't show Oceanic there. There is a route for one of the lines Liverpool-Queenstown-Quebec though.
I also thought I'd be able to find the border crossing from the US to Canada by Janet Brown and her brother for the wedding, but not on Ancestry. There only seems to be the reverse journey for 1900. But I can't find John M Brown returning.