Yes, I've seen that "sonne in law" reference, but it seems to be negated by the following record:
From "Deed and Will Abstracts of Stafford County, Virginia, 1686-1693," Part I, page 16: "Pages 19-19a. Bee it knowne by these presents that I Henry Blagrave of New Kent County in Virga. for a valuable consideration paid by John Gowry of Stafford Parish within ye County of Stafford doe fully discharge ye said John Gowry & have granted unto ye sd Jno. Gowry all my right and demand in land to Six hundred acres of land within the sd County of Stafford and by Pattent granted unto Capt. David Mansfield MY LATE GRAND FATHER deced beareing date ye 6 day Oct: Anno 1654 To Hold the sd Six hundrd acres of land to him ye sd John Gowry his heires and assignes for ever. In Witness whereof I have hereunto put my hand and Seale this 11 day of Octr 1686. Signed: Henry (HB) Blagrave. Signed in ye presence of us: Richd. (R) Wood, Will. Ballard."
The "sonne in law" reference was made in a deposition by Thomas Gregg, NOT by Mary herself, so it's certainly possible that Gregg made an error or misunderstood what Mary had said. The fact that Henry Blagrave II specifically identified David Mansell as his grandfather would seem conclusive that Mary Mansell was indeed Henry II's mother. I would assume that Henry II would state the family relationships more accurately than someone who apparently wasn't even a close family friend and was reporting on a conversation he had while visiting Jamestown 8 months earlier (the conversation he was testifying about took place in Oct. 1686, and he made his deposition in June 1687). Also, Henry II's line continued to use the Mansell/Mansfield given name in following generations, which would be unusual if Mary was a step-mother instead of his actual mother.
Do you have any info that Henry Blagrave I married widow Ann Harwood besides the vague reference I've seen in "Adventurers of Purse and Person"? Have you seen the actual records referenced in "Adventurers of Purse and Person"? I haven't myself, but another Blagrave researcher has and says that the records are open to interpretation and don't positively state that Henry married Ann. I've also yet to find any other records connecting Henry I to anyone in the Harwood family, widow or otherwise. If Ann had minor children and died shortly after marrying Henry, why isn't he mentioned in any continuing records concerning the children's estate, which would be customary? If Henry DID marry Ann (which I'm still not convinced of), I don't think she was the mother of Henry II for the reasons above. I know that Henry II wasn't mentioned as a grandson in David Mansell's will, but since Henry II would have been the oldest son of Henry I and Mary, he would have by law inherited his father's estate, so David might not have felt the "need" to take care of Henry II in his will (for this reason, eldest sons were often omitted from wills, since the law made it unnecessary by already guaranteeing that they'd be the primary heir).
As for Henry Blagrave II's wife Sarah, I suspect (but can't prove yet) that she was a Ballard. "Ballard" is used as a given name in subsequent generations; a William Ballard witnessed Henry and Sarah's 1686 deed to John Gowry (and the accompanying power of attorney); and in 1719, a Matthew Ballard willed a suit of clothes to Henry Blagrave (this could be either Henry Blagrave II or Henry III, since both would have been living at that time). Henry III had to have been born in 1685 or earlier, since he was old enough in 1706 to sell land he had inherited from his uncle Mansell Blagrave. In Oct. 1734, the vestry book for St. Mark's parish mentions a burial for Henry Blagrave. This was most likely Henry III, since he would've been about 49 or 50 then (Henry II would've been about 83). I have no clue as to Henry III's wife's name; his son Henry IV was the one who married Mary Knewstep and became known as "Henry Blagrave Sr." in Lunenburg Co. I'm descended from Henry IV's son Henry V, who married Elizabeth Stokes.
I have quite a bit of documentation on these generations, so if you'd like to send me your postal address, I'll be happy to send you copies of the info I have.