Using 'town' in its modern sense, that is, an urban settlement, there is no town anywhere in Scotland called Cambusmore.
'Cambus' is from Gaelic 'camas', and means a curve or bend in a river, and can also mean a bay. 'More' is also Gaelic, and means 'big'. So Cambusmore means 'big bend' or 'big bay', and there are very probably numerous places so named all over Scotland. These may be individual farms, crofts or houses.
The important thing to note is the
parish where an event took place. If the baptism was recorded in the parish of Duthil and Rothiemurchus (which is indeed in Inverness-shire), and the record just says 'in Cambusmore', you are looking for somewhere
in that parish or very near by. Not 100 miles away to the south in Stirlingshire, or 100 miles away to the north in Sutherland. Given that the River Spey flows through the middle of the parish of Duthil, and that it meanders as it does so, it is very likely that there was somewhere known as Cambusmore along the river somewhere there. I think that any other Cambusmore is likely to be a red herring.
However you have two pieces of evidence which contradict the likelihood of James Macpherson and Janet Cameron being the parents of your William: one being the death certificate and the other this reference to his birthplace being Sutherland.
The IGI at
www.familysearch.org says that William Macpherson and Margaret Heron were married in Edinburgh on 13 November 1825, and two children are listed: Isabella, 1826; James, 1828.
At FreeCEN
http://www.freecen.org.uk/cgi/search.pl I can see that the 1841 census listed Wm Macpherson, 40, warder in prison, Margaret, 30, and James, 12; Christian, 10; Alexander, 8: Margaret, 6; Thomas, 3; and infant, 5 days.
In 1851 William, 52, born Sutherland, is living in the Prison of Edinburgh with wife Isabella; and children Christina, 19; Alexander, 17; James and William, both 10; and Jane, 7. I suspect that James aged 10 ought to read Thomas aged 12. I wonder if Isabella is in fact his second wife?
I wonder whether there are any staff records from the prison that might shed light on William's origins? I would start with the City of Edinburgh Archives
http://www.edinburgh.gov.uk/cityarchivesI also see from the IGI that James Macpherson and Janet Cameron had a fairly large family, and that the first two were born in Ardersier. That immediately makes me think of the Army barracks at Fort George, and makes me wonder whether James was a soldier stationed there. But as you can't prove they're yours, there's no point going looking for James' service records yet.