I wonder if that 1846 baptism is the correct John McIver?
In 1861 John is listed as aged 9.
The John McIver, son of John McI and Catherine Kean, in the Catholic birth records was born in 1846 and would have been 14 on the day of the 1861 census.
A five-year discrepancy in a much older person's age would be slightly unusual, but surely even the most arithmetically challenged parent would be unlikely to get a child's age wrong by such a large number?
Apart from this, there is a family in Glasgow in 1851 consisting of John McIver, 40, born Ireland; wife Catherine, 33, born Ireland; and son Hugh, 9, born Coatbridge. Hugh's age exactly matches that of Hugh, son of John McI and Catherine Kean, who was born on 18 December 1841.
However there is no sign of Hugh's younger brother John, born in 1846, who would have been 4 on the day of the 1851 census. There's also a sister Mary, born 28 April 1844; Scotland's People indexes her mother's surname as Carn but I read it as Cain on the original. She would have been 6 on census day 1851.