HERE is an interesting paper from Osaka University in 1985 describing SAT behavior. A few things jump out at me.
One, phase separation isn't quite as big a deal as I thought. While guaranteed grade SAT quickly drops to 160J/g latent heat then stabilizes, technical grade SAT with 1% Tetrasodium Pyrophosphate Decahydrate (used in toothpaste and cat food), quickly drops also but stabilizes at 200J/g. The trend line shows that it is unlikely to degrade below that. Adding polyvinyl alcohol, acetone and liquid paraffin as thickeners kept SAT stable at 230J/g practically forever.
Two, many nucleating agents work, but none will until first forced into crystallizing the SAT, apparently by lowering the temp. All can be caused to fail by overheating (don't sustain any temp over 69C, and don't take above 79C at all), or by poisoning due to contamination with certain compounds. AC, I wonder if this might be why your Glauber's salt didn't work as described?