Hello Mate,
I think you would be in a minefeild with the legality of your plan,
"fit for purpose" and all that jargon etc. MOT tec.
You would also have a bit of difficuly in actually "fitting" the extra seat in the space available,
Changing the RR seats to LR seats is not what it will need,
You will actually have to move the LR seat outwards to try and get extra space for the third seat,
the other real problem might be the hand brake leaver.
this would be between the third person's legs, as would the G/B tunnel and could be uncomfortable on long run.
The Disco as mentioned is a good idea, or even get a large roof rack for the Shogun.
Cheers............