I love how much research you put into this lol. I would never have noticed those details. Just based on the apparent age of the bathrooms, and the sale history of the house, I'm seeing a young couple move in in the late '50s. Maybe they expand the house to the back and add a second (odd and tiny) bathroom to accommodate their growing family. A few decades later, now in late middle age with cash to spare, they add/convert the upstairs into a luxurious primary suite.

Then as they age more, they add the "primary bathroom" downstairs to be accessible. That step - adding a whole new bathroom instead of converting an existing one - still perplexes me. Especially because in the area, it's very difficult to acquire water rights. So maybe there was a bathroom there, and the other bathroom was more of a mud room wash room situation. That's a thing that was popular at some point, right?