score:8
As far as I am aware, I am eligible for a refund of my entire ticket due to this flight cancellation
Correct
However, could I also opt to only fly the return journey, and get (roughly) half of my ticket value back?
That's unlikely to happen since LH would have to reprice the ticket which is complicated and will NOT result in a 50% refund. I'm guessing your options are to either change the outgoing flight or cancel the whole thing. If in doubt, call LH and ask.
Have you considered taking the train instead? Deutsche Bahn operates the IC bus to Munich and there is plenty of trains from there to Nuernberg. You can probably do the whole thing in about 5-6 hours for less than 30 Euro each way.