Specifically, how does the extra hey in devarim teach that the Shema may not be read out of order?
Avraham Sacks, Ramat Beit Shemesh
The added Heh implies a restriction ("the" is specific), from which Rebbi learns that the "Devarim" must be left as is. That is, it is not permitted to read it out of order.
All the best,