I'm not aware of a daughter named Mary for William Spence and Eleanor Draper. The only daughter I know of was Eleanor Spence, who married John Quigley. According to a 1695 deed, William Spence left a will that was dated 7 Sept. 1692, but I don't think the will survives.
I haven't seen the postings by Barbara Beall. There's a 19-year age difference between Patrick and William, and for 2 people of the same surname who lived in a tight-knit community, they conducted their business in entirely difference circles -- not what you'd expect of brothers. I tend to thinkt they're cousins, or uncle (William) and nephew (Patrick). I'll have to check out Barbara's posts.