i found some biographies that have been added to familysearch in the past year or two. we'll need to dig up the primary sources but i think it's highly likely that these will lead us in the right direction.
here's a summary (based on information from the biographies): our ancestor (robert bell ross sr.) had an older sister named mary bell. she was born in 1815 in west calder to john bell and mary paterson. she married walter muir in 1835 in west calder. two mormon missionaries (elders samuel mulliner and alexander wright, natives of scotland) found walter and mary, and the family was baptized in 1848, when mary was eight-and-a-half months pregnant with their seventh child. around that time, mary and her siblings added "ross" to their last name. so far, no evidence has been found as to why this occurred. from mary's younger brother's life story (which we need to find), there was an "uncle" robert ross who apparently paid for the schooling of the bell children in edinburgh. there may have been some legal adoption by this "uncle", or perhaps something happened to the father, john bell, and robert ross was made legal guardian. we don't know. whatever the reason, even the married children added "ross" to their name. mary's two younger brothers carried on the ross name. their mother, mary paterson, was listed as a widow in the 1841 census, and was found in the 1861 census at her daughter mary's home. our ancestor, mary's younger brother, robert, was baptized by walter muir in 1850. mary's tenth child, elizabeth wright, was born around 1856, and was apparently named after robert's wife.
i believe that this can lead us to the evidence we need to show that robert ross, who married elizabeth wright in 1854, is the same person as robert bell, born 1830 in west calder to john bell and mary paterson.
here are links to the complete biographies:
Walter Muir and Mary Bell (Ross) Story by Carma Muir Golding
(Sil) Addendum to Walter Muir, Sr. and Mary Bell Ross
Biographies of Walter Muir Sr. 1809-1860 and Mary Bell (Ross) 1815-1888
so let it be written... so let it be done.