Hi mweston....welcome to the frustrating (but sometimes rewarding) search for Sutherlands in this part of the country
Unfortunately, I have not got any of these Sutherlands in my tree. So sadly, no I don't think we are related...as far as I can see anyway.
You may not ever get a birth/baptism for William Sutherland being born around 1854. Registration only started in Scotland in 1855, so before that you have to rely on the child being baptised...and from my experience in Caithness baptisms are few and far between (with my Sutherland's at least!).
I can see a tree on Ancestry that has the NZ marriage record for William Sutherland & Isabella Kelly attached (perhaps this is your tree?) showing the below information:
Marriage 1 Aug 1882, Lyttelton
William Sutherland, 28yrs, bachelor, tailor, b. Caithness, Scotland, parents Alexander Sutherland (shoemaker) & Jane Sutherland (MS Sutherland)
Isabella Kelly, 24yrs, spinster, b. Hobart, Tasmania, parents William Kelly (Labourer) & Isabella Kelly (MS Farquhar)
The census returns you found in 1861 and 1871 at Torbeg, Dunbeath Estate seem likely to be 'your' William Sutherland, especially with his being listed as an apprentice tailor in 1871. Is there any other confirmed link to Torbeg/Dunbeath or the grandparents David & Margaret Sutherland? Do you know whether David & Margaret Sutherland are the parents of Alexander or Jane? Both William's parents having the surname Sutherland makes it a bit more of a challenge!
Do you have a maiden name for William Sutherland's grandmother Margaret?
Women in Scotland are often recorded in death records after 1855 under both their maiden and married names, so you might be able to track down his mother or grandmother's deaths searching on Scotland's People that way.
Torbeg appears to be quite a small settlement, it is referred to in Scotland's Places in 1871 as consisting of 3 crofts.
https://scotlandsplaces.gov.uk/digital-volumes/ordnance-survey-name-books/caithness-os-name-books-1871-1873/caithness-volume-06/281I can see that the census returns show the below:
1861 - Torbagge, Latheron, Caithness
David Sutherland Head 64yrs Farmer b. Latheron, Caithness
Margaret Sutherland Wife 62yrs b. Latheron, Caithness
Margaret Sutherland Dau 33yrs Domestic Servant b. Latheron, Caithness
Mary Sutherland Dau 20yrs Domestic Servant b. Latheron, Caithness
William Sutherland Grandson 7yrs Scholar b. Latheron, Caithness
1871 - Torbeg, Dunbeath Estate, Latheron, Caithness
David Sutherland Head 75yrs Crofter b. Berriedale, Caithness
Margaret Sutherland Wife 73yrs Crofter's Wife b. Dunbeath, Caithness
Margaret Sutherland Dau 43yrs Crofter's Daur b. Dunbeath, Caithness
Jamesina Sutherland Dau 32yrs Crofter's Daur b. Dunbeath, Caithness
Mary Sutherland Dau 29yrs Crofter's Daur b. Dunbeath, Caithness
William Sutherland Grandson 17yrs Tailor (apprentice) b. Dunbeath, Caithness
Jamesina Sutherland Granddaughter 6yrs Scholar b. Dunbeath, Caithness
The 1881 census is a bit of a mess on Ancestry, but from what I can gather there still seem to be a number of Sutherland's living in Torbeg (around 10). You could buy the image on Scotland's People to see what it really says, but it looks like David & Margaret may have died and a couple of their daughters are still at the croft?
1881 census - Torbeg, Dunbeath Estate, Latheron, Caithness
Margret Sutherland Head 53yrs Crofter b. Latheron, Caithness (speaks Gaelic)
Mary Sutherland Sister 40yrs b. Latheron, Caithness (speaks Gaelic)
So you have quite a few names there to track down. I wonder if Jamesina Sutherland is William's sister or cousin? It certainly is a more unique name to try and track down than William. Though I see that on Scotland's People there are still two Jamesina Sutherlands born in Latheron Parish, at about the right time one in 1865 and one in 1866...so you would have to take a chance and pay for the certificates if you wanted to chase that one further.
What a fascinating story, and an intriguing puzzle to try and solve
Jenny