The bit that may be interesting to see in court would be if the Westjet flight was a codeshare, rather than an interlined ticket. There is a recent ruling that had Westjet done something wrong in the London->Halifax direction then BA is now liable. So it stands to reason that this judgement may work the other direction too, so Halifax to Europe. But you weren't on a codeshare so it's a bit moot. I can't see any other ground for WS being pinged for this by EC261, and as you noted Canada is introducing some limited consumer protection in this area.

Moreover, all EC261 would have done in your case would be to cover the hotel and food bill until there was a flight rearranged by Westjet + BA. There is no automatic right to another service on another airline on a different route. And had you stayed in Halifax I suspect your insurance would have paid the hotel bill, therefore I'm not sure this is adding a lot.

But hopefully your case is an eye-opener to others, and indeed will perhaps encourage people to look at their insurance - or indeed self insurance - and run some realistic scenarios through the small print to see if the outcome is acceptable or not.