I'm not so sure it would be quite so huge an issue - it depends how merc did it.
Iirc the cars without had a single large under floor cavity. My car has the 7 seat option and the seat squab lifts out entirely and the back hinges from the floor behind the squab.
If the mounting slots on the sides of the cavity are there or could be fitted and you could get the seat hten maybe. You do have to add 2 more seatbelts so it depends if the mounts for them are there.
Not easy certainly but might be worth checking. POst some pics of your boot and under the boot floor and it may be easier to tell.
Ah yes, I think you might be right :fail on my part.I don't think this is what he was asking David. The OP wants to know if he can change the fixed rear seat in his saloon to the drop down folding seat(s) which is an option at build level.