Yes, I wrote that biography. I was maybe a bit too tactful, but I wasn't sure what resistance I might face from other descendants, and I've gotten myself into some fraught disagreements in the past. It looks like I needn't have worried so much here. :) I will certainly flesh out the research notes and competing parental theories as I work on this.
Contemporary sources are certainly the biggest problem we're facing here. I suspect that may be why the Maryland theory took hold, because there -is- a christening record for a William Wellborn there, who would be near the correct age to be this one. There are no birth records in the fitting time period in Virginia that I've been able to find. Land records might be the best place to look for clues, but I am not experienced with that type of research.
The AJC article was published late, yes, but it came from William's great granddaughter. The same tree was published in a work by a granddaughter's husband. Those generationally close sources have to carry more weight I would think, but of course, they aren't as good as contemporary sources would be.
I haven't done much work at all from Samuel back, because I've been stuck at trying to prove William's parents. He does have more sources on other sites though, including a will at Ancestry, assuming it's the correct Samuel. Interestingly, if he is William's father, that would make William several years older than we currently have him, since that Samuel died in 1728.