Gudetama

Your thesis can still hold, because it is kinda odd that Mio was at the right place at the right time. I mean... it took Hiroto a while to remember the mall, so it is not a common venue to hang out - which makes it very strange that Mio was there. That said, X did meet Mio later in episode 2 as well.

Mio met X around the 33rd minute in episode 2. Remember when Hiroto went to the police station to pick up Michito, Mio went home. But X followed Mio and then approached her when she parked her car. Very subtle scene.

I think she is called "mama" (short for mama-san), because she is the owner of the restaurant/snackbar. I don't think it was used affectionally, but more like a title.