So sorry to hear this happened and you were led astray for the past two months when the GUC process was initiated. The agent that started it should have caught the issue then. Generally an "X" fare would not be an issue had it been purchased directly with Delta. Since it was booked with Viking it was a bulk fare and the system mapped it to X, which is one of the lower economy fare buckets, but still carried the underlying terms/conditions with it being a bulk fare.

Best advice I can give is to never buy an airline ticket except directly with the airline. Things can get very messy, very fast.

Only possible idea I have is to buy two economy tickets directly with Delta and then reapply the GUCs in the hopes they clear. I realize that probably isn't practical if the trip is close in. Fares are likely to be high and no seats may be available. And I doubt your tickets through Viking are refundable, so probably not an option.