Hold ON!
I checked the Arbory burial records and discovered that the Thomas COSTEAN who was buried 23 Jan 1834 was 12 yrs old

There is a Thomas Costean age 67 who was buried 22 Apr 1840 Arbory

In 1841 Arbory Census
2 12 50
Edwin Gawne 6
Ellenor Costain 40 this is Eleanor Cannell Costain
Ellenor 12
Elizabeth 20
John 15 Thomas 15

2 12 49
COSTAIN
James 25
Ann 3 d/o Henry & Elizabeth Cubbon
Jane 25
John 70
John 45
Martha 65

2 12 51
COSTAIN
Thomas 55 This is Thomas Costain who md Esther Bridson 30 Nov 1793 Malew
Margaret 40
Richard 40 ch 26 Oct 1812 Arbory

So I think your theory is still alive and well.

Donna