I agree with you 100% about a relationship b/t Catherine (Taylor) Addy and Dr. Joel P. Taylor.I would bet his father, William, to be her younger brother.
I am sure there has been more work done on the Taylor line -- just not by me .
Have you seen the bio on William Taylor originally published in 1895 in "Memoirs of Georgia?"
These Lutheran families from Lexington District, SC seem to be well researched.I would try checking with one of those counties websites or genealogical societies.They were good Germans and wrote everything down.It's just our nature (I am about 70% PA Dutch).
Let me know if I can help you further -- I recognize your name and believe we have corresponded before.