There is protection against N1/N2 overspeed on the B737 NG
Not entirely correct....N1 overspeed protection yes but with AT ON only....N2 protection with AT ON AND OFF.
To avoid over thrust with the AT OFf, the Ng has a great feature which is predictive thrust ,as you advance the thrust levers you will see on the N1 display the N1 you will get for the TLA position...therefore allowing crews to correct a possible overshoot before the ACTUAL over thrust occurs.