I ended up splitting the post in 2 parts - one part of my search for her as someone raised in the Newmarket-Lake Simcoe area of York County, and the second part because I'd found her autobiography, Ringing the Changes where she describes her publishing career, and I realized what an inspiration she could be to other writers.
Although my Mazo de la Roche Part 1 post is about Mazo's personal life, I don't mention the 2 children she adopted while living in England. But while researching the Newmarket Era for my 1911 Courtship Letters on my Author Memories blog, I found this snippet: