A very fun, if frustrating, mystery! If it was my little mystery and I was very sure it wasn't my family, I think I would do a couple things. You said you looked at the census, but maybe try again and concentrate on the streets close to where your own family resided. If the photos were together they must be friends.
If the photos aren't related at all, then I think I'd look on line for an historical society that would cover the Minneapolis area. I would look to see if they have a forum where you could post this same question. You might have luck there.
If you've done that, I'm sorry. But that's what I'd do, and if it didn't help, I'd have to live with the mystery. :)