What I found interesting is that they claim no clear mechanism of action but still mention this Nrf2 pathway.
Hypothetically, do you suppose dietary changes to enhance the Nrf2 pathway would be additive in respect to this medication or overkill and possibly dangerous?

That's strange for sure, as even wikipedia claims that inducing the Nrf2 pathway is the probable mode of action for this drug.
As for how how targeting the system with dietary factors would interact with Tecfidera, this is impossible to predict - but would make a very interesting scientific study. As from my personal lab experience, I saw both synergistic and antagonistic interactions of compounds with similar modes of actions.
But for sure I would be careful not to overdose Nrf-2 inducers, for the reason of hormesis.