I went back to the thought that we both agreed on which was to not modify a factory voice. Tried a quick test to see what would happen:

I made a simple change to one of the factory voices and rather than save it as a user voice I overwrote the factory voice. Next I tested the "button" labeled restore factory voice, and it did work so you can "get back" to the original voice should you decide to change one. Here's what I didn't expect - Even though I modified and overwrote the original voice when I went to a style and selected that voice the style still plays the original factory sound - not the modified factory sound that I "overwrote."

In summary what you said appears to be true - You can modify a voice and either overwrite it or save it as a user voice. You can select and use that "new" voice as a right-hand voice - What you can't do is use that voice in a style, even though you assign it from the factory sound location. Hmm...