Wow! You must have spent hours on this. You didn't have to go to that much trouble. Thank you very much.
I had found some of this, but had not found the people in El Monte and the May and Becknell and Stowell connections. I have been working on a spreadsheet with censuses for all 12 of the girls with Mary as a first name (of any surname) in Tulare in 1860 who were born in Texas and about the right age - thinking, perhaps, our Mary might have been adopted by someone in the wagon train.
There are some Mary A. Browns elsewhere in Texas in the 1860 census but the only non-Lamar one that I haven't disproved already is one in Bexar where I can't find her (or the other members of her family) in 1870. And the Lamar wagon train story of Cornelia Glass is just too close to the family story in my wife's family for it to be a coincidence.
Clearly I need to order some microfilms from LDS...
Thanks again for all your help with this. I will definitely merge what you have done into my spreadsheets! This kind of collateral research is the only way to solve these kinds of problems.
Don