With regards to Harriet Selina, what a shame you hadn't mentioned her on this thread too as it may well be that her baptism is indeed at Hintlesham, as William's was, and could also have been checked. I note that one online tree shows a date of 6/10/1810 Hintlesham suggesting that it did take place there (occurring pre 1813 when Hintlesham baptisms not online).
Possibility for son William - cannot see him in 1841 when William and Maria are living at St. Margaret, Ipswich. In 1851 a William Bruce, Sergeant in 91st Rifles, unmarried, 29, born St. Margaret, Suffolk was at Dover Castle Barracks. I cannot see in other records a William Bruce born in St. Margaret's, Ipswich. If William and Maria moved to Ipswich shortly after their marriage to St. Margaret's parish perhaps young William thought that was where he was born.
Cannot find any further trace of this William so perhaps he was indeed the one who went to South Africa. Certainly the 91st were in South Africa in the 1850's. Something to keep in mind anyway.
Annette