19th: Dora Myrtle &/ or Dorothea Beatrice?

Those two lone FAIRBAIRNs in Brant's database that I mentioned the other day have revived my curiousity about Dora Myrtle FAIRBAIRN.
Brant kindly provided Dora Myrtle HART nee FAIRBAIRN's obituary, which reinforces that, as her marriage cert shows, she thought her name was Dora Myrtle.
I've placed a post-it against Dora on the online Rootsweb database of Rita in the hope that someone may be able to shed some light on this mystery (Rita's 2003 email address bounces, but there appear to be quite a few living descendants, so someone out there might have an answer)

I'm interested in what anyone may be able to tell me about the family of Franklin HART and Dora Myrtle FAIRBAIRN of Fredericksburg, Ontario.

I'm working on a theory that Dora Myrtle is actually
Dorothea Beatrice, dtr of Thomas James GOULD and Agnes Bush FAIRBAIRN.
Agnes died 3 Dec 1886, and dtr Dorothea Beatrice appears with her father Thomas in 1891 (as 5 yr old Dorothea B) and 1901 (Dorah D 15 b Jun 27 1885).
If she and Dora Myrtle are one and the same, how come she also appears with Mary nee STRAIN in 1891 (as dtr Dora FAIRBAIRN, 4) and 1901 (as "something" dtr Dora FAIRBURN, 13 b 27 Jun 1886 or 1887).
The "something" may be an abbreviation for adopted?.

I can't find any other way of explaining Dora Myrtle.
Her marriage cert to Franklin HART shows her parents as James FAIRBAIRN & Mary STRAIN, ie Agnes Bush FAIRBAIRN's parents, but she patently cannot be the dtr of Mary FAIRBAIRN nee STRAIN, although she could be her grddtr.
The puzzle is how come a Dorothea Beatrice can turn into a Dora Myrtle, and show up in two places on two separate censuses.

If they aren't one and the same person, who on earth is Dora Myrtle (the Myrtle only shows up on her marriage cert to Franklin, and her obit as far as I can tell) who appears to share the same birthday and month, and probably year, as Dorothea Beatrice.

So, I'm hoping for some family knowledge and someone who may be able to explain all this.
My interest is the FAIRBAIRNs. I've been trying to find more proof of my assumption of where Archibald James FAIRBAIRN, married to Mary STRAIN, fits into the picture, and one way of doing that is to find descendants who may have either family stories, or be willing to represent his line in the FAIRBAIRN dna project (he himself appears not to have had sons, so his particular line is a literal dead end as far as dna testing goes, but someone might be in touch with cousins).