Forget about suing them. Company employees and companies are protected when their employees make mistakes.
Also, Sweden isn’t litigious as American culture is and you’d only be able to sue them for actual damages anyway.
You’ll have to try and use your points to book a new trip, and then submit claims to customer service for any additional costs beyond what you would have paid otherwise within reasonable limits. When they refuse to reimburse you, you can submit a case to ARN.
I’d suggest you just book the SAS ARN flight that you got an award seat on unless your return trip is still valid.
Also, it’s very clearly stated in the booking rules that you can’t change a reservation once you have used the outbound trip. From SAS perspective you’re trying to pull a fast one and they caught you.
I refuse to accept it's my fault. How can it be me pulling a fast one when I call customer service and their trained employees tell me "this seems a better option, can I change the booking to this flight?".