Full disclosure: I've been working with HEC from LotionCrafters for a few months, but I've only just figured out how to make it work in LS (at least, make it work for me!). I'm far from an expert, but I may be able to help you work through the process.
First, the HEC from LotionCrafters has a built in hydration delay to prevent "fish eyes" when adding it to water. As a result, it's best to hydrate it in cool distilled water for about 30 minutes. At that point, it should be slowly heated (I use a water bath) while stirring until the temperature is about 75C. At that point it should be nice and thick, and can be incorporated into your formulations. To avoid lumps at this point, I've found that it's best to slowly stir your other ingredients into the HEC-thickened water. Trying to stir the HEC-thickened water in other liquids (IME) causes lumps.
The problem with adding the powdered HEC to liquid soap, is that a high pH (soap) causes the hydration to run too quickly. Just like you, I've tried taking diluted LS, adding the HEC (mixed into a little water), and heating it all up. The result is a thick and clear soap--until it cools. All of my attempts have ended up with the HEC settling to the bottom of the container, and still leaving the soap cloudy.
My best advice (until I "perfect" my method), is to pre-hydrate the HEC in water, slowly heat it to thicken, and then add an already diluted soap. LotionCrafters states that the usage for HEC is 1-3%. If you over thicken your water, you might be able to add just enough of it to your soap to thicken it. My experiments, thus far, are suggesting that 0.5% - 1% HEC is sufficient for soap.
I hope that helps.
