I have also located the family in the 1861 Census at which time Mary A Rowe gives her age as 22 and birthplace as St Austell.
Phillack appears certainly out of the equation although it is possible there may be some connections.
Mary Ann was therefore born about 1838 and would have been about 20 when she married James Rowe.
That marriage is registered at St Catherine's House in the September Qtr of 1858 so I think the date you have been supplied will be incorrect (1st January 1858?).
The 1851 Census shows 'a' family of William and Mary Northey at Phillack.
Amongst their children is a daughter Mary, age 12 - so she would have been born about the right time (1838/9).
However, all members of this family are listed as having been born at Phillack.
If we go back to the 1841 Census I think you may find that this is your family:-
1841 Census - St Austell
William Northey, 30, miner, in County
Mary do., 25, not in County
Catherine do., 9, in County
William do., 6, in County
Maryann do., 2, in County
In this case, the mother was not born in Cornwall so it may take some effort to find the marriage to William Northey.
There is an interesting sideline here.
I found a record in the St Blazey Parish Register when I was in Cornwall in 1994 which shows the marriage of a Matthew TREWELLA to a Jane NORTHEY 25th December 1853.
Both parties were then 'of St Blazey'.
I believe I have connected Matthew to the section of the Trewella family he should belong but Jane is another problem.
Her father's name was given as William and her age indicated she was probably born sometime after 1833.
I have found record of two probable children for them - both registered in the Liskeard R.D. - but can find nothing further on this family after 1858.
Now I turn back to the 1861 Census to have a look at the entry for William O Rowe and his family.
1861 Census - St Blazey, Bridge Road
William O Rowe, head, mar, 51, tin miner, ST STEPHENS
Mary do., wife, mar, 52, St Just
Richard do., son, 15, tin miner, St Just
If this is your William Osborn Rowe then he says he was born at ST STEPHENS.
(Problem is - St Stephen in Brannel or St Stephen by Saltash
)
Son Richard is now aged 15 which means the information supplied to me by Elizabeth Rowe Tobie may be a little doubtful - birth date given as 28th July 1848.
In 1871
St Blazey - Fore Street
William Osborne Rowe, head, mar., 61, dock labourer, St Levean
Mary do., wife, mar., 64, St Just
Jane E do., g'daur, 12, scholar, St Blazey
Thomas do., g'son, 8, scholar, St Blazey
About the best I can do with this just for now so will throw it back to you to think about.
Now, as for the death of Mary Ann Rowe (Northey)
1864 December Qtr. St Austell R.D. Vol 5c Page 94 - Mary Ann ROWE
This is probably her based on the information you have supplied.