If adoption wasn't legislated until 1921, then earlier forms were informal; and hence, no records. After speaking with some colleagues about this subject, it was felt that those that "adopted" BHC did so as if they would make that child their own, whereas most children were indentured simply because the farmer/household wanted cheap labour (which is how Child Immigration was promoted by/to some). I am told that many "adopted" children kept their own names, but some did indeed take on the adoptive parents name.
I have also been told that some children had their names changed just so they could not be traced. There was a time when BHC were disliked by society, being referred to as "street urchins". Also, few (any?) children coming in the 1880's had documentation. I think you're right about Ada's birth date issue.
If you look at Ada Scott in the 1901 census, there is a notation in the "Relation to Head of Hosehold" column: it's difficult to read but FamilySearch transcribed it as "Cs Daughter". I believe this is in error (no disrespect to the indexer's; Lord love them for doing a great job!): I believe it is a stylized/handwritten "A" which denotes "adopted daughter". I have seen this notation in two other census returns.
Tracing this family back I find that Duncan Scott was previously a farmer, and always seemed to have a hard man; but does not seem to have had any other children. Again it makes sense that if Duncan and Margaret couldn't have children that they would be inclined to adopt a child, Ada being the lucky recipient. However, going back to the 1891 census (trying to build my case) I don't see Ada, but an "Edie Trigg". Is this Ada with another name? I see a 12 year old girl from England with another strange notation in the relationship column. Could this be a complex census error/issue combined with an obfuscated name? I like to think so.
My fear is that the only record that could clarify this is Annie MacPherson's records, (a) which may not exist, and if they do (b) I understand I am not priviledged too (because I am not a direct descendant).
p.s. re: Ada Clark, I think I'm on the wrong track. My Ada had a sister Mary Ann, who had daughters named Ada and Irene. It was Irene's marriage registration where I found the witness named Ada Clark. I'm thinking now this was Irene's sister; not her aunt.
p.p.s. I am in Ontario (Waterloo Region). I think my next step is a drive to Stratford to see if I can find an obituary for Ada (Scott) McDermott.