As it seems like you realize, there are 4 Little League charters in the B/CS area - 3 are in Bryan (National, United, and Harvey) and then there is all of College Station. I, too, was under the impression that only College Station was doing fall ball, so I was surprised to see United is as well. When we've done fall ball (we are with National), we had to combine with United to have enough kids to make teams and our games were mostly in College Station. As noted, fall ball is instructional and league boundaries don't matter.
During the spring season, boundaries only really matter if you want to play All-Stars. These boundaries can be based on your address, OR the address of the school your child attends. For example, if you child attends school in National's boundary, he/she can play at National regardless of where your home is.
All of that said, I'd imagine there are Bryan kids signed up to play fall ball in College Station, but, yes, you'd likely have the majority of Bryan kids sign up to play with United. Would those kids play with Harvey in the spring? Hard to say.