So it's been awhile since I had the 25FB, but if I remember right that heating ducts ends under that bench. Couldnt you just shorten it to whatever new length the bench/couch would be? Or maybe just cap it off. If you cap it, the other vents would probably just blow more. Ours also had the base speaker under the door side bench, but I would think that could just sit behind the recliner.
I don't know how much cold weather camping you're gonna be doing, but we rarely use our furnace. When we have power hook ups we generally use a space heater if needed. It's much quieter and doesn't use our propane
We save the furnace for when we are dry camping.
I am surprised that the dealer didn't have any good ideas to offer you.
You may have better luck with a forum member that has a resoration shop that specializes in Airstreams. He's a great guy and does top notch work. Check out Area 63 Productions, his name is Uwe. The shop is in Orange, CA.
BTW, if you're ready to attend your first Airstream Rally, we are having one in two weeks just a few hours from you. Check out the link at the bottom of my post