Sometimes the Running back is a little in front of the QB, sometimes a little behind, and sometimes in line with the QB. Why do they do this? How does the change in the Running back's position in relation with the QB change the play? What situations would each be called for?
Running Back Position in Shotgun
it could be blocking scheme alignment or that is the angle needed to run the zone stretch or zone read.
Think of it as being in an "I" formation and the fulback could be offset or go in motion. It is all done to get the correct angles for blocking or getting to the correct holes. ("Thats what she said")
can "guess" the play based on where the RB is lined up, or based on where he turns/ moves right after the snap? This would not be good.
Not any more than having an offset fullback in the I. Think about it. The RB can follow the FB, run the opposite way, they can pitch to the RB either way, 3 step drop pass, 5 step drop, quick out, bootleg, wheel route, etc. The same goes for our current formations. Just because the RB is lined up a certain way means nothing about the play that is being called.
You're right. You could line up 5-wide and run the ball. Or you could go full house double tight and send everybody deep. But there are things called "tendencies." Like when Michigan goes "end over" it has been run 100% of the time. And when Michigan lines up in the "I", it's usually a run. And when Barry Sanders used to play for the Lions, whenever he lined up as an offset single back, it was almost always a pass. And when you see an I-back lined up 7 yards off the ball, it's usually a run. Etc. Etc. Etc. You can do anything out of any formation, but generally there are reasons for where players align themselves.
I have seen this as well, but I don't think the running backs' alignment tips off the play much. I'm sure they change their alignments for a reason, but it's not necessarily "they're passing if the running backs are deeper" and "they're running if the running backs are up." I've seen them run and pass out of both alignments.