Please, please don't go down the road of 'there is only one possible candidate in the records so it must be the right person'.
There are many people whose births, baptisms and marriages were simply never recorded. If you latch on to this Margaret Steen born in 1829, you may be climbing a branch of quite the wrong tree. Or you may not, but you need more evidence to be sure.
In particular, the surname Steen could possibly be an abbreviation of 'Stephen' or 'Steven'.
The Margaret Stein who married John Robertson in 1839 cannot possibly be the one baptised in 1829, as she would not have been old enough to marry just 10 years later. I had no trouble finding the marriage in the church records on SP, by the way.
The 12-year-old Margaret Stein in Currie in 1841 is in a household headed by Thomas Stein, 40, and including Isabella Stein, 35 and six children aged 15 down to 1. None of these children's baptisms are indexed on FamilySearch (Scotland's People is offline at the moment) but it does not look as if this Margaret could be the daughter of John Steen and Janet Black.
John Steen and Janet Black had a son William, baptised in Bo'ness in 1831. I have not found him in the 1841 census at
https://www.freecen.org.uk/cgi/search.pl so I am wondering if perhaps John, Janet, Margaret and William had left Scotland before the 1841 census.