I genuinely wonder if and how that would hold though, you need the buck to stop somewhere, if Alberta were to vote to leave Canada you may call it illegal as you want to they won't just say "this is treaty land" and cancel their own referendum.

Say they separate politically and renege on the treaty, the first nations will go to the ICC? Or ask Canada to invade its own province? What support if any would the later have with the Canadian elector, sending the army to fight against other Canadians?

It's very similar to the old constitutional argument that separatism needs a "clear majority" which sparked questions that following a "yes" in Quebec the supreme court would have to statute on whether 51% is a "clear majority". Would Quebec actually have just accepted a ruling against them from a institution that is not really theirs?