Thank you, Loralee!
Yes, her husband is also missing. He might, however, be the Thomas Sword referenced in the 1870 Lovell's city directory as living at 183 Centre Street.
Lovell's Directories [1] 1870-1871 page 444 ↑ https://stevemorse.org/montreal/montreal2.html
If so, that is the last time he has been spotted in the records using his LNAB. By 1878 he is marrying again, in Goderich, using his middle name (which is also his mother's family name) as his surname. This might have something to do with being a deserter from the British Army, or it might just have been easier (for him, not for me, since Robertson is a very common name).
He seems to have stayed in touch with Aunt Mary White, at least enough for her to know that he had started using Robertson instead of Sword as his family name. My grandfather knew that Robertson was his mother's maiden name.
Thomas may have been related to Aunt Mary White, though i haven't found the exact link. They both have MacDonald ancestors. Thomas' brother Andrew married a girl from the Glengarry community, however, so maybe she found Aunt Mary White to take in the toddler.
(Mary) Theresa Marsh might have been less connected to the Glengarry community. Or, she may have come from there, and fancied her name up a bit herself. She might really be just Mary or Mary Theresa, with Adelaide being her own fanciful addition.
I believe that the time lapse in getting the baby christened might have something to do with Aunt Mary White being Roman Catholic. As you know, in those days the churches did not accept each other's sacraments. If little Janet had been christened already, it would have to be done again to make her a Catholic.
The children of Thomas' second marriage knew about the child of his first marriage, but Janet and Edgar died so young and my grandfather ran away from his uncle so young that if he had been told he had uncles and aunts on his mother's side, he didn't remember.
I have tried to look through the church registers in Montreal and the whole 1871 census, looking for either Sword or Robertson, and I haven't seen them yet. I also think it is interesting that little Janet was born about 1874 when Thomas and Theresa married in 1869. There should be an older child, even if it died young.
Thanks so much for being a fresh pair of eyes....