Thanks to the hands-on efforts of Lu Grubbs a Chancery Court lawsuit has produced some pretty solid evidence that William Douglas did not marry the (3 known) mothers of his children. A Bill of Complaint, filed by Wm Douglas' "Ross" children (lead plaintiff was Gabriel Lee Cauly/Cauley/Corley, husband of Sarah "Sally" Ross) claimed that the Administrators of the Douglas estate (who happened to be some of the "Griffith" children and their spouses) were dragging their feet in distributing the real and personal property. Here is a quote from the Complaint, which is not contested in the Answer:
"The said William Douglass although he was never married left a large number of illegitimate children all or most of whom were objects of his bounty both in his lifetime & under his will."
It is also interesting to note that his six "Ross" children and 6 "Griffith" children were born during the same range of years, more or less alternating between 1810 and 1822.
Margaret "Peggy" Douglas (eventual wife of Fountain/Fontaine Morrison) was Wm Douglas' first child, with Mary Noffsinger. Fontaine and Peggy also sued the Douglas estate Administrators over the withholding of personal property (in this case, a slave promised to Peggy). That case went up to the Virginia Court of Appeals. Once again Lu has done the research legwork. I'm in the process of transcribing those case documents and we'll see what additional revelations will come out! The Appeal describes Peggy as William's "favorite daughter."
As far as I have been able to find out, none of the mothers married anyone else, either. My g/g/g/grandmother Elizabeth "Betsy" Ross pre-deceased William by about 7 years. I think (according to recitals in the lawsuit) that the Ross children continued to live on the portion of William's estate they would eventually inherit. I don't know who looked after them - six kids between 8 and 18.
I'm happy to provide copies of transcribed Court documents to anyone who wants them. (Note about Gabriel Lee Cauly - in his Complaint, Gabriel Lee spells his last name Cauly. In the other documents, including Decrees and Answer, his surname is spelled Cauley.)