"My goals: additives that are cheap and common which make SAP work indefinitely or at least 1000 cycles with little degradation, backed up by practical experience..."
You realize that these goals are at odds with each other, don't you? A more predictable and/or stable formulation can cost as much as you would care to spend...
From what I have seen so far, it looks like the nucleating agent will be much more expensive than the acetate(if it's exotic), and maybe more than the emulsifier(depending on ultimate stability vs price). For your purpose of imposing a lower temperature boundary on the phase change, it seems this ingredient will be the most important to consider. The more rigid your spec, the more important this ingredient becomes.
I like the pyrophosphate, since it is a food additive. In case some pcm containers burst or leakage occurred, there would be no cause for alarm of contamination. Same thing with the cellulose gum. In fact, knowing that the TETRA form worked, I might try some experiments of my own with baking powder. Who knows, the stuff might actually work.
OTOH, if a nucleating agent exists that vastly improves phase change consistency that is not so benign, I might choose a more impervious sequestering agent. Much modern work has been done in this realm. When mixed hot, it has been proven that one can make jelly, rubbery or plastic solid polymers that remain stable pretty darn easily out of the acetate and nucleating agent. The stuff is being made into building materials today that can be cut, drilled, or nailed through with no measurable leakage or performance degradation.
Another thing I have found in general is that the organic(waxes and fatty acids) pcm's serve a better purpose as a blanket or jacket of some sort. In their solid form, the thermal conductivity is low enough to consider them as a layer of insulation. So as they cool to below their meta-critical temperature, they help hold the heat inside the container. The salt formulations don't insulate so well, so they are better purposed as you propose, inside the container.
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