By 1912, I think it unlikely that spouses would have called their spouse Mrs. or Mr. - that's a rather 18th Century convention. However, they may have so referred to their spouses in conversation, at least with strangers. I was always rather fazed by a friend's mother, who referred to her husband as "The General" in conversation, and that was in the 1970s, despite the fact that I called him "George". I think it was rather an affectation. In the same era, I also had a friend who married an Iranian princeling, whose servant used to refer to her in the local UK village greengrocer's etc. as "Her Serene Highness, Princess Jane". It was most embarrassing, especially since Jane had grown up there. The marriage failed.
In 1912, upper class British children often called their father's "Sir", as in America, and I would imagine it might be a convenient way of getting round what to call the bloke, especially if your natural father were still alive. That wouldn't have affected so many people then though, as divorce was comparatively rare. If the real father were dead, then I think it likely that they would address the stepfather as "father".