Hi Ella
Jenkin is a prolific name in Cornwall. They are everywhere. The Cornwall OPC site shows there are no Jenkin marriages or births at St Teath before Nathaniel and Mary. So they moved there. There is one death at St Teath in 1671 of Charels (Charles?). He could be related to Nathaniel.
The Robert Jenkin - Nan Trenance marriage is in the OPC database. Their source is Phillimore's. It took place on 13 Nov 1655 at Warleggan.
The John Jenkin - Paternell marriage is not in the OPC data base. I am suspicious of those dates. If they are Robert's parents either her YOB has a typo or she had Robert when she was ten. Also Robert's wife and mother have the same YOB.
There is no suitable George Jenkin (or variants) and Elizabeth in the OPC data base. The first George marriage is in 1632. So maybe early marriage records are thin on the ground. Have you asked the tree owners about their sources? It's hard to see how the tree owners have linked Nathaniel to Robert when they don't appear to have a DOB for Nathaniel. Maybe they have a Will.
I would forget those marriages and concentrate on finding Nathaniel's origins. You have no birth/baptism for him and unfortunately neither does the OPC database.
https://www.cornwall-opc-database.org/home/Good Luck
Venelow
Canada