Montreal has always been distinct from the rest of the Province of Quebec. It was the largest and busiest port in Canada at the time I was born (1957). MTL has welcomed successive waves immigrants: Norman French, United Empire Loyalists, the Scotsman who ran the Hudson Bay Company, Irish refugees (starting in 1840), Jewish refugees, refugees form every country in Europe and the Middle East, etc. After the British Empire collapsed (1962) waves of immigrants arrived from every former British Colony. The "Big City" was far more diverse than out-lying farming communities in Quebec. Montreal was one of the few cities in Canada with distinctive Jewish, Portugese, Italian, etc. neighbourhoods.
In the aftermath of Confederation and the American Civil War, all the "big money" moved from Nova Scotia to Montreal.
After 1867, Montreal was the first Canadian city to industrialize to build rails and rolling stock for the new trans-Canada railway. That railway opened up the Prairies for settlement and "bought British Columbia into Confederation. Prairie wheat had to pass through the Port of Montreal on its way to world markets.
Montreal declined in importance for two reasons: Saint Lawrence Seaway and rumours of separation. SLS allowed ocean-going vessels to by-pass the Port of Montreal on their way to the American Mid-West and its booming automobile industry.
The Quiet Revolution also started around the time I was born. QR started with strikes at asbestos mines, but soon overwhelmed Quebec society as the Catholic church was pushed out of the public sector to be replaced by government-run hospitals, schools, universities, orphanages, health care, pensions, welfare, etc. Quebec rapidly shifted from a feudal society to a modern welfare state. That shift was far too fast for old-style, pure wool families to retain control.
Rumours of an economic collapse after separation forced many large banks and corporations to rapidly move head offices to Toronto during the 1960s and 1970s. The 1970s and 1980s also saw massive amounts of investment capital shift to Calgary for the oil boom.
To make Montreal a distinct city-state/province, you need to hand-wave away the Saint Lawrence Seaway and the Quiet Revolution. Also limit major social changes to big cities (e.g. Montreal) while out-lying faring communities remain dominated by the Catholic Church and a near feudal economy.