Literacy and backstory

Yesterday I entered family tree information for the ROBINSON family into my Reunion database. Most of the wonderful things I have found about my extended family in County Westmeath, Ireland have come from either original Quaker records or from writings of the late Liam Cox, and this was no different. I found lots of information on Anthony Robinson's descendants (also Thomas ENGLISH's descendants, since Anthony married his daughter Bridgett) from an Irish Ancestry magazine article, written by Cox, archived in the National Library of Ireland in Dublin.

This morning I was looking over some of old Quaker documents that I copied during my research trip. A few entries weren't about my family, but I copied them because members of my family signed as witnesses and I wanted their signatures. I had noticed before, at a wedding, that Dinah English Clibborn signed with "her mark." But at that particular wedding, the handwriting is all the same (copied into the book by the clerk) and lots of people added their marks, including John Clibborn.

But this morning I saw a list that contained different handwritings (original signatures), and again, Dinah made her mark. And so did John Clibborn, and John's sister and a few other people. Which leads me to think that John Clibborn did not know how to write. That blows my mind (and my mental backstory), because John was wealthy, English, and an only son. But he was also a former soldier. So, even though the first son usually inherited everything (and the second became a soldier or clergy), the lack of education makes me think that John was not wealthy as a child. Perhaps he saved his army wages and then shrewdly sold better lands given to him in the Cromwellian settlement and bought cheaper land in the country - in Moate.

John and Dinah Clibborn's niece, Ann English, and her brother Thomas could write their names. In a previous post, I mentioned that, although I don't know where the orphaned Ann and Thomas lived, my book carries the logical conclusion that John and Dinah took them in. With no regular schoolmasters in the area during those years, I assumed the children were homeschooled. But, if neither John nor Dinah could read or write, who taught Ann and Thomas (and the Clibborn children)?

Here's a synopsis from four events (1670s) with signatures:
John English could write, but his sisters (Dinah, Margaret and Bridgett) could not. John's second wife Eleanor and Bridgett's husband Anthony Robinson could write.
Neither John Clibborn nor his sisters Alice Tuart, Ann Miller and Bathshebah England could write. John's brother-in-law Phillip England could write and Thomas Tuart made his mark in earlier years but apparently learned to write his name later.
There's also a mysterious Mary Clibborn that wrote her own name on several documents. She is not one of John's sisters or daughters and, as I said, he's the only son. Who is she?