This is what we got from the genealogist Ellen Lindsay. She also sent us attachments to prove her point!
I've spent a fascinating time this morning researching your Rennies. It is my considered opinion that William Rennie 1821-1896 was never married, and did not father any children.
My evidence for this comes from a number of sources:
We know for sure from William's death registration in 1896, that his parents were Alexander Rennie and Ann Robb, confirming his birth as 1821 Elgin. On that death registration (you have a copy from yesterday) William is noted as "Single". Had William been married this would have been highly unusual, although not impossible if his wife had predeceased him by many years and contemporary friends did not know her (but it was a family member, cousin, who notified his death).
None of William's many death notices and obituaries in the newspapers make any mention of a spouse or children. I attach a couple of the newspaper obits, and you can see that other notices on the same pages as William's note spouses and/or children if there were any.
I have found William boarding in Kensington on the 1881 census. Many single officers and retired officers took rooms rather than have the expense and encumbrance of having a full household in town. William is noted on that census page as being "unmarried". (census copy attached)
As we know from William's probate records, he left his possessions to his sister Agnes, appearing to confirm the lack of a spouse or children.
According to deployment records of William's regiment, it is extremely unlikely that he was in the Elgin area in the 1850's to meet and marry and have a son. His parents and siblings had also moved away from the area up to Sutherland long before then, probably around late 1820's,as some of his siblings were Sutherland born.
Moving to the other side of the argument - the marriage in St Andrews-Llandbryde Parish of William Rennie and Margaret Innes, who produced a son Robert 5 months after their marriage in July 1854.
It is my contention that this is a different William Rennie. The William Rennie who married Margaret Innes and had a son Robert was much younger, having been born in 1833.
On the 1861 census at Moss of Barmuckity, we find William, a Railway Labourer, aged 26, with his wife Margaret, and son Robert aged 6.
This William is confirmed as the son of Robert Rennie and his wife Helen Laing who married 30 Jul 1830 in Elgin. William was born 19 Jun 1833.
Living almost next door to William, Margaret and young Robert in 1861, also in a house at Moss of Barmuckity, are his parents and youngest sister Robert and Helen Rennie and young Isabella.
Going backwards, to confirm the family group, on the 1841 census, William is living with his parents and siblings at Moss of Barmuckity. (I have attached copies of all of these census details for you)
Following young Robert forward... Robert was born on 20 Dec 1854. As noted above on the 1861 census he is with his parents. On the 1871 census, he is still at Moss of Barmuckity, age 17, but living with his grandparents Robert and Helen.
Robert jnr (b.Dec 1854) married Mary Younie in St Andrews-Llanbryde Parish in 1879, and by 1881 is living at Old Shields, St A-Ll, with wife Mary and a daughter Margaret W Rennie aged 1. His occupation is Cartwright. He goes on to become a Carpenter later.
Robert and Mary go on to have a further 2 children, John and Mary Jane in 1883 and 1885.
Then things start to take a turn for the worse for the family - in 1886 and 1887 Robert is bankrupted (newspaper articles attached). In 1891 and 1901 censuses Mary his wife is alone with the children and is making a living working in the local woolen mill. In 1890 and 1896 Mary makes application for Poor Relief to the parish. We would have to get the detail of those applications from Elgin, but it would tend to indicate that either Robert has died, or gone and left the family destitute. I can so far find no trace of a death, so suspect the latter for now!
Mary finally dies in a nursing home in Elgin in 1932, with her home address being Maisondieu Road, Elgin.
There is no record of any other Robert Rennie having been born in the area around that time, so my contention is that he is NOT the son of your William Rennie - and that the Robert Rennie buried in Lucknow is a sheer coincidence.
I suspect that someone in Elgin has put 2 and 2 together and made 5.