When you've been doing genealogy as long as I have (almost 30 years) one thing you learn is to think outside the box.
As already stated, the 1861 census shows that George Steven born St. Cyrus was a servant to William and Agnes, not a relation, so they are more than likely not connected to him at all. This was in Dun where his family were. The 1851 Census shows the only George Stephen born circa 1842 St. Cyrus is yours. You have to be flexible on surnames the further back you go. Records were only as good as the person writing it down and most people couldn't read or write and had no idea what was put down. Since the 1861 Census shows only one George Stephen/Steven born ca.1842 St. Cyrus - in Dun where his father was - then I would say this is definitely your man.
Don't reject entries because the spelling is not what you expect. With the 4 other children you refer to (as in 1841 census) the eldest, John, could have joined the army, emigrated, moved to England. As to the 3 girls - they could have moved to England (no trace in Scotland) but then Scottish christian names can change like the wind. Jean's can become Jane, Janet, Jessie, Jeannie, Margaret's Maggie, Meg. Then again, maybe they died like their mother.
Annette