Yes, the race for 1st is over. The only question left is who will finish 2nd.
If NW can win the NW-Neb game, they will have a big advantage, going to 4-2, and having beat both Iowa and Neb. Neb still needs to play PSU and Iowa still needs to play OSU and Wisc. So if NW wins, the race for 2nd is over.