Your problem has to do with the couch that came in the Sovereign
. The seat of that couch just pulls up, pulls out, the back falls down and it makes down into a bed. Airstream installed a better couch in the Excella and Limited models. On that unit, you grab the back of the couch and pull it up and over the seat which remains stationary. The back pivots all the way over and ends up in an upside down position toward the rear of the trailer. Sleeping on that couch is generally great. As you've found, sleeping on the one you have can painful.
I occaisionally see couches for sale here on the forum's classified section. There was one listed not long ago by itself and it is possible that it might still be listed as "for sale". There was also a gentleman who was wanting to sell a complete interior. It seems to me that his couch was one of the better kind. If you can find a couch out of a Limited or an Excella, you will likely solve your problem.