After the incident that I shared at the top of the thread, I researched it extensively.
A passport is not required by Canada or USA for entry if you have a Nexus card.
A passport is not required by Air Canada if you have a Nexus card according to their website. There is one place where it is "recommended" but not required.
Further, the Air Canada tariff affirms that if you have all legally required documentation and they do not let you are board, you are Denied Boarding.
So, it is not required.
But, having been through the ordeal that I've been through, I would bring printouts of their website pages, the US CBP and Canada government immigration and customs pages, and more, as the gate agent who denied my family boarding was insistent and intransigent. e.g.
https://www.aircanada.com/ca/en/aco/...documents.html