I recently had a similar kind of difficulty rebooking (needed to change, so cancelled, went to rebook, used google flights to set routing, but could not get FFC to show up at checkout). What I wound up doing was just booking the new ticket with credit card, then calling up and giving the agent both records to reconstruct on the original ticket and cancel/refund the new one. I think having the new itinerary fully ticketed helped, and overall it did take about half an hour, but it went relatively smoothly. Not ideal having an extra $1.5k on the credit card for a few days, but they took away the pay at airport option so that was all I could see to do. In your case, if I was getting some something about "illegal" itinerary, I'd have been inclined to say let's leave it alone and then call back to try again — probably a sign the agent / price desk didn't know what they were looking at.