I have not yet found a baptism for Mary Ann Daniel but it looks like the following could be her:-
1851 Census
Treggaminion, Morvah
Thomas DANIEL, head, mar., 35, tin and copper miner, Morvah
Elizabeth do., wife, mar., 30, Madron
Elizabeth J do., daur, 8, Madron
Thomas do., son, 7, St Just
MARY A do., daur, 3, St Just
Betsey do., daur, 1, St Just
But 1861 provides a problem.
'A' Mary Ann Daniel age 8 (born St Just) is with her uncle and aunt Thomas and Eliza WHITE.
Thomas White married Eliza Daniel 20th May 1850 at St Just.
Eliza was d/o Thomas Daniel and birthplace in 1861 recorded as St Just.
But then there is a Mary Ann Daniel age 13 and born Sancreed who is a servant at Madron.
This last one might be the one from the 1851 Census.
In 1871 Mary A Daniel age 22 and born St Just is a servant at Penzance.
Again this is probably the same Mary as in 1851.
But then there is Mary Ann Daniel, age 19, born St Just who is a servant to her cousin Thomas Ellis at Boscaswell, St Just.
This is probably your girl.
Thomas Ellis age 23 of Boscaswell (son of Thomas) married Grace Williams age 19 of Trewellard (d/o John) 16th November 1861 at Pendeen
This then makes Mary Ann a daughter of the following marriage:-
Thomas Daniel age 24 of St Just (son of Thomas) married Mary Ann Ellis age 20 of St Just (d/o Thomas) 26th July 1845 at St Just
This might be a bit confusing
but I hope it helps solve part of the problem for you.
I will look at it again tomorrow and get my database updated.
CT