I hope your relatives are feeling better again, and your trip has contributed to them making a speedy recovery.
Thank you for writing this, it was an interesting read and I had to chuckle quite a number of times at your remarks, although I feel sorry for your hassles.
There is something wrong with the way your EWR-GVA transfer in YUL works. If you arrive at YUL from the US, and connect internationally, you would not be required to go through Canadian customs I believe.
But yes, once it became clear that you would have to spend the night in Montréal, you obviously had to immigrate into Canada, THEN pass Canadian customs with your bags before exiting the airport.
Too bad you did not use your forced one-day stop in Montréal to go downtown and discover this beautiful city!