There is nothing in the contract of carriage or in the law that impose on anybody to take any flight. In addition the prevailing law in Germany allows out of sequence use of tickets. The fiction that a flight from A to B, change the airplane and the flight from B to C is one flight is just a fiction and most courts will see them for what they are: two flights.
In fact the whole charade is organized and orchestrated by airlines. It is not negotiated to any degree.
The Lufthansa claim of fraud is as bogus as it gets, regardless of any merit of the largest European German airline lawyers filling in a wrong court in their home country - where are the damages? The airline most certainly cannot show that the airplane burned more fuel because it was lighter? Or can they prove that they did not seat anybody else in the empty seat?
How about price gauging by the airline pricing two flights cheaper than just one of the two flight? Where is the discussion or ethics or law on that?
Anyway - last year I bought 9 Lufthansa tix. Lufthansa defrauded me on 7 of them and refused to correct. When legally challenged - Lufthansa provided copies of documentation from different year, with different pax names and different routing.
On the other hand out of many instances of flight cancellation/irregularity LH was the only airline painless in honoring the EU-261 obligations