I read somewhere that trở lại means you're returning to a previous state/place. As in, you went to a shoe store, left the store, and returned to the same shoe store. I always thought of trở lại as pivoting. Whereas trở về means that you're returning "home". Your house, your hometown, your home country, whatever. Whoever explained that in the comments of another question really tuned up my understanding. It totally makes sense why I would choose one or the other! =)
trở = to turn, to change
lại ~ a particle with lots of different meaning. After a verb it means again.
về ~ also a particle. I don't know it's exact definition in the context, but it can mean "to return", "to come back" , "about" or "on"
So based on your post, i think, lại/về is a particle that specifies more of "trở"