Well, I can't say I see any confusion myself.... According to my notes commander Frederick Welstead was born sometime between 1786 and 1791 (your date may be correct and I may be wrong on that), resided at Stoneley Cottage, St.Neots and died 1848. He married Emilia Sophia Bristow at St.Peters, Isle of Thanet on 22nd November 1813. She was baptized in Bengal on 13th January 1785 and died at Stoneley Cottage on 4th July 1857.
They had (at least) one son Frederick, and two daughters of whom the younger died unmarried and the elder married her 1st cousin George Emilius Hannam in 1847.
Emilia Sophia was, as your source rightly says, a daughter of John Bristow (c1750-1802) president of the board of trade at Calcutta. Her mother was Emilia Wrangham (see my original post in this thread). This John Bristow was probably born in London and baptized at St.Botolphs, Bishopsgate on 15th January 1750, and it is possible he may have spent some of his childhood at his fathers home at Quidenham, but he seems to have spent his entire working life in India and died there in 1802.
It was, as your source rightly says, his father John Bristow (1701-1768) who owned Quidenham Hall, apparently between 1740 and 1762. This John Bristow was active in the South Sea Company, was a member of parliament, and is sometimes called a Director of the Bank of England. He died in Lisbon in 1768.
It is highly unlikely that Quidenham Hall was still owned by John Bristow (senior) when he died in 1768, and this fits in with it being sold in 1762. John Bristow (senior)'s eldest son, Henry, became a Lieutenant-Colonel in the Coldstream Guards and resided at Dover Street in Piccadilly. He died in 1786 at Bath in Somerset leaving a son and a daughter... the son, George (c1765-1833) appears to have had no connection with Quidenham Hall either, working and dying in India.