Alberta being created out of the NWT (Rupert’s Land had already been annexed by Canada decades before) has no bearing on what constitutional amendment formula would apply in case of succession; the Constitution Act 1982 makes no distinction. The Clarity Act though does say that the provinces should be included in the negotiation of the constitutional amendment granting succession. But it doesn’t give an opinion on whether their consent is necessary (ie which amendment formula to use). So yes succession could be under the unanimous consent formula or the 7/50 formula but it could just as well be under the “amendment to the Constitution of Canada in relation to any provision that applies to one or more, but not all, provinces” formula where only the House of Commons, Senate, and relevant province’s legislature need to consent. (The Quebec Succession Reference Question affirmed that a province’s membership in Confederation isn’t just part of that province’s constitution meaning provinces can’t just amend their own constitutions to unilaterally succeed)

And yeah since treaty are affirmed as part of the constitution by Section 35, the constitutional amendment granting succession would also require renegotiation/amendment of treaties 6, 7, 8, and 10, which would in turn require the consent of the federal government and all the party First Nations. Maybe you could argue that if Alberta stayed a monarchy then the Canadian Crown could pass its responsibility to an independent Alberta Crown the same way the Imperial Crown gradually became an independent Canadian Crown, but I doubt the courts in this day and age would just ignore Indigenous protests to that. Especially considering that the Crown had been represented by the Government of Canada for the entire time the Numbered Treaties have existed and all but Treaty 7 would then have to deal with the Crown splitting in two.