I have found a baptism for Rees Nanney in Dolgellau in 1688. I can't be certain that he is the one who married Elizabeth Coulston but he is the right age. However, there is a problem because he was illegitimate.
I had missed him on previous searches but Dolgellau has a printed version, which was clearer than the films of the Parish Register and Bishop's Transcripts. Both are in Latin and translate as: -
Rees the illegitimate son of Evan Nanney and Margaret David, concubine - Baptised 21st February 1688
I have not found a marriage for either parent and so it is difficult to know if he always used the surname of Nanney.
This is possibly his father's baptism: -
Evan the son of Griffith Nanney and Katherine - Baptised 19th August 1666.
Evan Nanneys found in the records: -
Several Evan Nanneys in Maentwrog or Ffestiniog. Probably from the Cefndeuddwr line. An Evan Nanney died in Kerry, Montgomeryshire, in 1722. The bond following his death mentions Elinor Nanney.
No marriage found for Griffith and Catherine but there are several Griffiths of that period: -
Griffith, son of Hugh Nanney and Jonet Salesbury. Pedigrees show that he was unmarried. Griffith, parents uncertain, born c1630, married Mary Scawen and died in Horton, Buckinghamshire, England, in 1704. Griffith, son of William Nanney and Ellin Edwards, born c1635 - no marriage record found. Griffith, son of Edward Nanney and Elizabeth Gwyn, born c1610 - no marriage record found. In 1642 there is a baptism in Dolgellau which is only partially legible but looks like Griffith son of David and Jane
Mentioned in Hearth Tax of 1662 Dolgelley - Gruffith Nanney (Two Hearths) Dolgelley - David Nanney (One Hearth) Nanneyuwchavon - Gruffith Nanney (One Hearth)