Dan,I think the reason you didn't find a family tree on Ancestry for W.A. Hildreth is because the person who posted the tree listed his birth year at 1855.
https://www.ancestry.com/family-tree/person/tree/44501679/person/24050875552/facts
For me,the painting looks nothing like Wyatt Earp. It list no provenance.Oil paintings are often faked to look older than they are .