[a-priori]

My point is that legally the First Nations have the right to not consent to the separation of provinces from the country. Of course, it's always possible for parties to act illegally...

If Alberta did unilaterally declare independence (which would be illegal according to Reference Re Secession of Quebec [1998]), the First Nations have the right to call upon Canada to defend their treaty rights under the "peace and good order" terms of the treaties.

If Canada did grant Alberta independence without First Nations consent, or Canada refuses to defend their treaty rights, they would have a claim that Canada had violated their rights to self-determination under the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP) which Canada ratified in 2021. But UNDRIP is a non-binding resolution, so I don't think they'd have a case with the ICC or ICJ (even assuming it had jurisdiction).