Does UA fly FRA-DEN?
I thought this route is only served by LH. Most likely OP was ticketed in Premium Economy.
Hence, OP has the right to get 75% refund of the FRA-DEN segment. Air carrier surcharges are included in the refund, as well.
For the next time:
As the original flight was cancelled, you have the right to demand a reroute, which has a timing of your convenience (subject to availability of physical seats).
You had to right to refuse the reroute via ZRH, SFO - without forfeiting your ticket.