The Constitution Act is just a piece of legislation; it can be changed if there is political will to change it.
I would be in favour of dropping the treaty idea completely and just giving some aboriginal-titled crown lands back to FN and treating them like real sovereign Nations. With the caveat that they lose Canadian citizenship (and their Status cards, our military protection, the rule of law, democracy all sources of canadian funding, education, health, currency, etc.). I mean it would be total chaos... but isn't that what they're asking for?
First, the Federal constitution is not just a piece of legislation (as opposed to the BC constitution which is just a piece of legislation). Amending the federal constitution is not easy and requires the consent of those that are effected. Treaties with First Nations can not be unilaterally altered by the feds and even if one were to change the federal constitution the treaties would remain in full force.
Next, the constitutional and legal history of Canada starts in 1763 with the Royal Proclamation. The Proclamation is clear that the Crown is required to settle with First Nations before any land can be used for other purposes. On the prairies this was done through the various numbered treaties. In BC it was not done but land was still settled. We have a very messy situation in BC and is in no way the fault of the First Nations but sits 100% on the Crown being unwilling to fulfill their fundamental duties under the Royal Proclamation and then imposing the Indian Act without any consideration of the reality of First Nations on the ground.
The problem for the Crown is that they need to show the legal mechanism by which they control the land. Canadian jurisprudence does not recognize 'conquest' as a legitimate way for the Crown to gain title.
In the north west of BC the assertion of sovereignty by the Crown was done before any British officials knew what existed. There are First Nations that had no ongoing contact with government till after BC joined confederation. The first step should have been a settlement with the First Nations but this did not happen. In the early 20th century BC First Nations were pushing hard for settlements with the Crown. The response from the federal government was to make it illegal for First Nations to work towards a settlement with the Crown.
The Wet'suwet'en have been in the treaty process for over 25 years. The Feds could have settled all of this by 2005 if they had ever bothered to take the issue seriously. In Quebec the Feds settled modern treaties much faster. With a settled treaty this would not be an issue at all.
I know First Nations people in BC that do not consider themselves to be Canadian citizens because there has been no agreement with the Crown. Many of them do not vote and do not claim benefits from the government.
With the Wet'suwet'en there is also the ongoing problem of who should be allowed to speak for them.