You are due compensation for the cancellation, and refund of any reasonable expenses you incurred dealing with the chaos of the reroute.

Google flights does not come up with any options for getting to ORB from DUS on Saturdays and Sundays, so the initial plan was probably to get you to ARN, and then get you on a taxi or coach the rest of the way That would make sense.

But the absence of ground crew, and details of the further plan, is pretty pathetic Though I would probably have tried to talk to Lufthansa in Frankfurt, and see if they could give more information. I think they still handle SK in Frankfurt. Though if not they might have a bit better connection to SK than the airport handling company in DUS.

I would raise a claim for comp and incurred expenses to SK customer relations And I would add description of the flow as well. Short and to the point of where they failed....