Asking Some Adoption Questions

My great great grandfather, James Muir, immigrated to the U.S. in 1887. I have been researching my Muir ancestors from Scotland for the past several months as I promised my father I would write a book about family history this year. When Dad worked on our family's genealogy, he had no real way to access Scottish records. I thought writing a book about ancestors he knew little about would be more interesting for him.

I absolutely love the ScotlandsPeople website and have attended several classes at the Fairfax Genealogical Society's Spring Conference and the National Genealogical Society's 2014 Family History Conference about researching Scottish records. I also participated in the Family History Writing Challenge, writing about the Muirs, earlier this year in preparation.

A few days ago, on Tangled Roots and Trees, I wrote a post about discovering my first cousin three times removed and his wife Anne "Annie" Hutton adopted two sons. Learning about the adoptions answered several questions I had, such as why I couldn't find them using the Muir surname on ScotlandsPeople, but it also raised other questions about their oldest son, Nathaniel Muir (1895-1908). So I thought I'd pose those questions here since I have access to so many people smarter than me about Scottish records.

I first discovered Nathaniel as the 6-year-old son of Nathaniel and Annie on the 1901 census. I could not find his birth registration. He was not listed on the 1911 census so I assumed he may have died and went looking for his death record. I couldn't find that record either. The 1911 census did list two adopted sons, Andrew Murray and Robert Stewart. Then I found a photo on BillionGraves.com of a memorial Nathaniel and Annie erected in Bathgate Cemetery. The inscription provided death dates for sons, Nathaniel and Andrew. According to the inscription, Nathaniel died on 19 Mar 1908 at age  13.

Portion of Nathaniel Muir McGregor's birth registration
from ScotlandsPeople

Monument created by Nathaniel and Annie (Hutton) Muir at Bathgate Cemetery;
photograph courtesy of BillionGraves.com

I went back to ScotlandsPeople and searched for any Nathaniel that died in West Lothian in 1908. By removing the Muir surname from my search criteria, I found Nathaniel's death registration, which listed McGregor as his surname. His name on that registration was Nathaniel Muir McGregor and his mother was Christina McGregor. With that information I went looking for his birth registration and found it, too. His name at birth was Nathaniel Muir McGregor. He was born on 25 January 1895 to Christina McGregor, an unmarried domestic servant. No father was named on the registration. He was born Whitburn, which was less than 3 miles from Armadale, where Nathaniel and Annie, his adoptive parents, lived.

When I found his other two adopted brothers, their birth registrations had been changed to reflect the name change at the time of the adoption. Nathaniel's birth registration had no such alterations.

Does this mean Nathaniel Muir McGregor could have been the illegitimate child of Annie's husband, Nathaniel Muir? Or does it simply mean Christina McGregor knew before her son was born that Nathaniel and Annie were adopting him? Or should I research Christina McGregor to see if she is related to the Muir line somehow? What should I think about this situation?