The to-ing and fro-ing across the river from the StSbyS area to Devon and back -- my people from that area did it at least twice a generation between c1815 and c1865, for instance. FindMyPast shows quite a few Kerley events in Devon in the 1800s-1900s. Possibly he was from Devon and buried there ...
(And the to-ing and fro-ing of bits of Cornwall itself: in 1841, one of my StSbyS ancestors was living in Maker, which was called Devon in that census.)
"Mary Jago (I think) was buried aged 72 born
1740 1750? on 22 Mar 1822 of Millbrook at Maker Cornwall."
"This ties in perfectly with Mary Henders baptism in Lanteglos." i.e. 1750, parents John and Eliz:
http://www.cornwall-opc-database.org/search-database/more-info/?t=baptisms&id=1493310John and Eliz (misspelled Elizebath for Mary's baptism) also seem to have had in Lanteglos
John 1742
John 1744
Margaret 1748
Do you think that Joseph Hender burial might be him and there was an error made in the parish records resulting from a mixup with Mary's family name, if they were related? Perhaps he was a nephew, son of an unmarried sibling of Mary ...
But ... FindMyPast shows two Joseph Hender marriages in Maker,
1765 to Elizabeth Cowling
1797 to Philadelphia Lander (son of first - baptised 1773 maker, parents Joseph and Elizabeth)
the first of which is most likely to account for the 1815 death.
(It looks like the younger couple moved away -- Philidelphia Hender died 1817 in St Endellion and Joseph Hender cordwainer of Indellion was in Bodmin jail in 1823 for a month for being idle and disorderly -- results of person search at OPC site.)
Something that might be worth looking into:
https://familysearch.org/pal:/MM9.1.1/J733-38Kbaptism Joseph Kerly, 1707 Maker, father John. A previous generation? There are other births in that batch C05287-1 to parents John and Hannah.
A John Charles Kerley was baptised 1865 Lanteglos by Fowey, parents John and Lavinia, residence Polruan. Don't see that birth or a marriage for the parents in the GRO index. Father was a railway clerk, maybe from away, but there's still no registration. In 1871 that child (John Kerly) shows as born in Plymouth, living in Polruan with Stephens grandparents. I suspect the parents weren't married - a spinster Lavinia Stephens married a widowed Thomas Bunt in 1881 in Lanteglos by Fowey. Lavinia was born 1841 Falmouth and is with her sister in Polruan in 1871. No clue as to who John Kerley the father was.
Just looking for any Kerley connection with Lanteglos or Maker or StSbyS and that is all I am finding!
Burials for StSbyS at FindMyPast seem to be only 1813-1837.
So Robert Jago could have been a mariner -- or if buried at StSbyS, the record just isn't accessible.
Thank you for your PM and offer, I will definitely keep it in mind!