Thank you so much for your reply, Barbara. I think it is strange that both wives were named Edith, so the first wife may not have been an Edith. Whomever Elizabeth's mother was, she was mtDNA T2 which came from the North-East area of Europe. Czar Nicholas of Russia was a T. The first wife may have descended from Hollanders as the family of Thomas Wilhelm were. Our family tradition claims Elizabeth Wilhelm was Pennsylvania Dutch, originally from Holland. Someone would have named a daughter after their mother which would be a clue as to who the first wife of Thomas Wilhelm was.
As far as David not being related to Abraham Slade, I have David (b. 1815) as Elizabeth's (b. in 1810) full brother by the first wife. Therefore David would not be a full brother to Abraham in any case, possibly a half-brother, however, probably the son of Edith's first husband, Mr. Slade. David's mother's mtDNA is the same as Elizabeth's and mine, T2.