I think Elizabeth emigrated to Canada as there is this death in the Inverness Advertiser 10 May 1873
" At London, Ontario, on the 15th ult, Elizabeth Cameron, relict of William Hitch, late of Dochfour, aged 77".
This would suggest she was older than shown in the 1851 census. She can be found in Canada in both the 1861 and 1871 census. I cannot see a death cert for her.
Presumably she emigrated with or to be with one of her children?
( There is also a death notice for William Henry Hitch in the Inverness Courier 16 Sept 1892
"At 7 Claude Villas, East Dulwich, London SE, on the 11th inst, formerly of Millerton, near Inverness".).
William