William of Spreyton's possible link to the Ussery family

View Between the Quay Gate and West Gate Outside the City Walls, Exeter by Francis Towne

As detailed in the previous post, DNA testing proves there is a link with the Ussery/Usery/Usry family. This is an even more unusual surname than Gidley. There is a distinct dearth of records found for any of the name variations in the Devon parish registers. Only one looks of any use to us - that of William Ussery, born on Aug 16, 1740 and baptised on Sep 21, the same year, in Dawlish, the parents being William and Sarah Ussery. There is no later trace of this family or the Ussery name in Devon. There is no record of the marriage of William and Sarah in Devon. Dawlish is on the coast, so was William Ussery born in 1740 the son of a sailor from another county or country?
Cindy H Casey has an Ussery family history website http://www.cindyhcasey.com where she lists several theories about the origin of the name. The fact that there are no records for it in Devon after the mid 18th century is interesting. The Ussery family were very early and prolific settlers in the southern United States, right back to the 17th century. There are no records for any of the variant spellings in civil registration in England or Wales (i.e. from 1837 onwards) until the mid 20th century. If William's father was descended from an Ussery, then it is also possible he may have been a descendant of an illegitimate Ussery line which carried the mother's surname.
To return to our William of Spreyton who married Wilmot Reeve in 1795. It is beginning to look as though our William was a Gidley descendant in the female and not the male line. When there are more DNA tests done, this may be clearer. But who could have been his mother?
Spreyton is nearer the Gidley base of the Winkleigh line than it is to those from parishes further south in Devon. So I looked there first. There were indeed Gidleys in Spreyton in the 18th century, descended from a brother of the famous Bartholomew Gidley, Charles II's supporter. If we assume that our William is the one in the Topsham burial registers in 1853, aged 87, of Countess Wear where he was a lodger in the 1851 census, stating his place of birth as "Sprayton" he was born in about 1766. There were, however, no female Gidleys in Spreyton of an age to have a child in about 1766. Except for one possibility. There is an entry in the South Tawton parish registers (the parish was close to Spreyton) for a William Gidley, son of Mary Gidley, in 1776, where the vicar has added an unusual note "son of Mary Gidley, his father having been transported"  The date of William's birth is given as 8th May 1773. Rather far out for our William, if we believe the census and burial dates. But I've never found a burial for this William. The date of birth is a huge stumbling block, though.
The documented Poor Law removal of John and Mary Gidley "with their three children, Samuel, Mary and Joanna, all under the age of 10" from South Tawton to Spreyton, where John Gidley was born and where his place of settlement would legally be, took place in 1772. Their son William was obviously not yet born, or he would have been mentioned in the Removal Order. And DNA evidence has already shown that our William could not have been John Gidley's child. The Winkleigh Gidleys are a completely different haplogroup.
One of the minor mysteries of the Spreyton tree was what happened to the older daughters of William and Wilmot Gidley. Their names were Sally, Ann, Mary and Elizabeth and were christened in Spreyton between 1795 and 1806. The youngest daughter Frances, born in 1810, is well documented. She moved to Heavitree, just outside Exeter, as two of her brothers did, and died there in 1858. Of Sally, the oldest, there is no reference after her christening. I think she may have died as an infant. The other three daughters survived to be apprenticed by the parish at the age of about 9 to local farmers in Spreyton. This was common practice to avoid a poverty stricken family becoming a burden on the ratepayers of the parish. I have recently discovered Ann, who moved well away from Devon after her apprenticeship, and married Thomas Linney in 1828 in Southampton. They had no children. Elizabeth I still cannot trace.
Going systematically through the Gidley burials in the Devon registers on FindMyPast I came across two burials I hadn't noticed before. They were both for a Mary Gidley, both in the parish of Holy Trinity, Exeter, and both lived at Quay Gate. The younger was buried in 1820 aged 21, and the older in 1823 aged 88. This made their dates of birth 1799 and 1735 respectively. It seems reasonable to suppose they were related. I haven't come across that address before in the Gidley database. The most likely relationship seems to be a granddaughter living with a grandmother. Looking at the baptisms in about 1799 with a grandmother called Mary, the only likely candidate seemed to be Mary Gidley of Spreyton, christened in 1800, William and Wilmot's third daughter.
Is it significant that William was found in the 1841 census at Countess Wear about 2 miles further down the Exeter Canal from the Quay? Could Mary's grandmother be Mary Gidley, the wife of John? The age tallied almost exactly, as Mary Blanchford who married John Gidley was christened in May 1736 in Bow, Devon, another nearby parish to Spreyton. I had originally assigned Mary a burial in Chudleigh in 1816. Her daughter Joanna Gidley was living in Chudleigh in the 1841 census and died there in 1846, and I thought it possible Mary had gone to join her, perhaps in her old age. But I had double booked that burial. It is more likely it is of Mary Gidley, nee Cassell, of a similar age, whose husband George had already been buried in Chudleigh in 1808.
There are some major problems:

• Our William couldn't have been John's child. 
• What happened to William Gidley born on May 8, 1773? He survived infancy to be christened in South Tawton in 1776.
• His age isn't correct for our William Gidley born about 1767. 
• Mary Gidley who died in 1820 in Exeter probably wasn't aged 21 if she was William and Wilmot's daughter, but 20. 
• Why were William and Wilmot living apart from at least 1841 until their deaths? 
• Which other female Gidleys were of an age and in a likely location to produce a child who was born in Spreyton in about 1767? See April 30 update below.
• Who was the Ussery descendant who was the male progenitor of William Gidley?

There are many unanswered questions, and nothing ties up neatly. But DNA has at least eliminated some possibilities.
April 30 update: I had forgotten a small tree descended from Bartholomew Gidley and Rebekah Smith. I have only traced their marriage, their burials and 4 children, but their son John christened in 1731 could be the progenitor of the Woodbury, Whitestone and Kent branch (John Gidley who was buried in Tedburn St Mary in 1789), or of the West Virginia Gidleys (John Gidley who was transported to Virginia in 1769).
Bartholomew married Rebekah in Cheriton Bishop in 1730. They seem to have moved to Tedburn St Mary by 1735 where the two younger daughters were christened. Bartholomew and Rebekah were both buried there, in 1756 and 1760 respectively.Their oldest daughter was Mary Gidley, christened in 1733, so of an age to be the surmised grandmother of Mary Gidley, a daughter of William and Wilmot. I have found no likely marriages for any of the daughters.