"...pH should be a good measure for safety as long as the lye is evenly distributed within your bar of soap..."
No, pH is
not a good measure of safety under the circumstances in which handcrafted soapers make soap. Or even under the circumstances in which large commercial soap makers make soap. If pH was the panacea of safety people want it to be, then commercial soap makers would use pH as a test, and they don't!
I will concede that if a soap maker could absolutely know the raw ingredients going into the soap were highly consistent, then pH
theoretically could be used once a soap maker established the correct pH for a fully saponified soap. That is not a realistic scenario. We don't and never will have absolute control over our raw ingredients -- the composition of fats varies depending on plant varieties, animal breeds, growing season variables, etc. and the purity of lye changes over time.
I've trotted out the table below some time ago, but it looks like it's time to trot it out again. The pH values of the commercial soaps range from 9.8 to 12.4 -- and I want to stress these are real lye-based soaps, not syndets (synthetic detergents). Each and every one of them would show a pink blush if tested with phenolpthalein, although the 12.4 soaps might blush pink and then go clear. Handcrafted soaps are no different, because they're made from the same lye and the same types of fats.
But enough. I've spent a lot of time trying to explain today -- especially Post 88 and this one. If my efforts are not meaningful to you, I am clearly not competent to fulfill this teachable moment.