Well, first, on your opening gambit, it is true that it is an act of sovereignty to negotiate trade treaties, but once you do that, they actually limit your sovereignty. They say that you cannot do this or you must do that. In the case of NAFTA, it is the required compensation. The Canadian government's sovereignty over walking away from that is limited. The Canadian government's sovereignty over throwing up tariffs outside the treaty is limited. Yes, you use your power to negotiate the treaty, but then the treaty puts constraints on the exercise of sovereignty.
As for property rights to natural resources, you can call them contractual agreements, if you wish, or you can call them property rights, but the fact is that if you and I have an agreement that I am allowed to use something or can receive a supply of widgets, in your case, for a certain period of time, cancelling that agreement without compensation is a violation of either property rights or contractual rights. You're quite right that in five minutes we don't have time to get into whatever fine distinctions there are between the two, so at this point, I'm quite happy to use the muddy terms “property rights” or “contractual rights”. We can work out the details of the definitions later.