I wonder whether you need to re-examine the theory that Harriet was Harriet BURRELL?
I suggest she may in fact be Harriet DACK, baptised 13 Jan 1824 at Tydd St Mary - parents Francis and Sarah.
Marriage 1: Harriette DACK, spinster, to William LEWIS, a silversmith, 20 Feb 1849 St Marylebone.
Marriage 2: Harriette DACK, “spinster”, to Henry HASTINGS, a labourer, 6 Apr 1863 St Luke’s Somers Town (St Pancras)
...
Hmmm. I like how the names fit. BUT...
I had ruled out Harriet Dack because there is a significant difference in ages... "my" Harriet Hastings was fairly consistent in her sources as being born 1832-ish (not 1824):
christening was in 1831 but in August so would appear one year younger in Censuses
1841 Census says 1831, Lincolnshire
1851 Census says 1832, Tyd St Mary, Cambridgeshire
1857 Marriage says 1832 (age 25, Spinster)
1861 Census says 1832, Gott Tydd, Lincolnshire
1871 Census says 1832, Tydd Saint Mary, Lincolnshire
1881 Census says 1831, Lydd St Mary, Lincolnshire - with son TFWL Hastings
1891 Census says 1832, Lincolnshire
1901 Census says 1823, Lincolnshire (9 years younger than previous age but in same household with daughter-in-law and grandchildren)
But how else to explain widow Harriet Smith being in her own 1881 Census (as someone pointed out in an earlier post)? Maybe Smith is just such a common name that it's someone else. And there were a number of Harriets born around 1832 in Tydd St Mary.
And the 1863 marriage source with Henry Hastings would fit an 1864 birth -- both in St Pancras. But there was no birth registration for a Hastings son with mother Dack, and no illegitimate birth of a Dack son (at least, not that I found).