Hi Viv
Out of curiosity I had a look at the 1871 and the son is John Thos but the father is just John Cain in the census - not sure if that helps in your thinking or not.
I do find the 1871 interesting in that the family has a visitor William Kelly born Marown same as Margaret, lending perhaps a bit of evidence that Margaret Cain is Margaret Kelly.

1871 Conchan
Ballachrink
Cain John head marr 33 farmer 70? Acres employing 1 man IOM Kirk Conchan
Cain Margaret wife m 33 farmer’s wife IOM Marown
Catherine Cain dau 6 scholar IOM Kirk Conchan
John Thos Cain son 4 IOM, Kirk Conchan
Alfred Cain son son 2 IOM, Kirk Conchan
Catherine Carran (?) serv unm 22 general servant IOM Jurby
William Morrison serv unm 21 general servant indoor IOM Santon
William T (?) Kelly visitor 40 shopkeeper IOM Marown