Yes, I have. The only thing I found is not to use old CP because it sucks water out of the MP and over time sort of sinks in. But I had great fun with it, and it worked well :)
Why would you want to include a silicone?
I would have thought it would make the bath / shower so slippery to be dangerous? I love it in barrier hand creams and some other products in small amounts, but not soap.
I had one batch of GM like that once, it was a bit like alcohol, not that pleasant. I was going to bin it, but stuck it at the back of my soap stuff. I dragged it out a year or so later and all the odor had gone. So don't throw it out, just get some more, and let the other one age.
How frustrating :(. I think the only way to know the answers to the weight questions is to write to Southern Soapers and ask them.
I only use percentages in my stuff so I get consistent results.
You are right "cups" can vary so much as can tablespoons. Cut your pain and ask them :)