Have you considered installing anchor's in the floor? That way, you can position the child's seat on the floor facing forward and properly secured. Anchors with 'D' hooks, properly installed, will secure the seat and fold flat when not in use.
The shoulder belt really isn't an issue at all. Every LATCH system is a 2-pt, without any 3rd point from a shoulder belt. Yes, there is a top tether, but not every vehicle can accept the tether, or accept it properly. The top tethers on some pickups I've owned are too close to the seat to even tighten it down.
You chose a good seat, though! Every one of my kids are in Britax seats from birth on. It is the ONLY seat I will buy - Britax puts a lot more technology in their seats than Graco or Evenflo, IMO.
I still think the OP needs to talk to the carseat manufacturer, or like another poster suggested, go to a carseat check. Everything else is heresay.
2012 Dutchmen Voltage Epic 3795
2010 Dodge 3500 DRW MC
2009 GMC 2500 DA CC
2012 Smart-for-two (sometimes hitches a ride in the Voltage!)
2005 Sundowner Sunlite 777
Check with the carseat manufacturer, also. AFAIK, none of the car seats can be mounted on a "side" position - facing the center of the vehicle. They are not designed to absorb impact properly in that position. If it were me, I would put the seat on either side of the dinette.
I pondered this and my 12-month old for quite a while. Car seats provide the most protection when mounted fore-aft and the vehicle is in a front or rear collision. The seats are also designed to provide protection in the event of a side-impact-collision.
Mounting a carseat on a couch in a C means that any front-rear collison will affect the child in the carseat as if he were in a side-impact-collision in a normal car. There are carseats designed with extra side impact protection.
The problem is finding one that will work without a shoulder belt (most couch seats don't have shoulder belts).
Here is a car seat with extra side-impact protection designed to work without a shoulder belt. http://www.britaxusa.com/car-seats/advocate-70-cs
The lack of a shoulder belt and a TETHER (4th mounting point) does decrease the efficacy of the carseat. Many developed countries will not allow the 2 point mounting acceptable in the USA.
In then end, after much thought, I'm leaning more and more towards a crewcab pickup and a trailer for safety reasons (the number one killer of small children in the USA is not disease - it's auto accidents). Side-to-side whiplash is much more serious than fore-aft whiplash. Better safe than sorry.
I'd have them de-activate the front-passenger-seat airbag(s) and put the seat up front or use two vehicles.