Hi Olly, I have your same situation
, 3 kids on board.
I bought also the extra seat, but I have installed it close to the door, so the access to the kitchen cabinet is easier and with view front of the street (opposite way of your picture).
My youngest kid is very still small, he is 16 months old, he is sitted with his car child seat on the bench so I can keep the single rear seat very close to the rear bench and so there is enough space for the legs of the oldest kid without move back the bench and with this configuration I can leave installed the bed with his net.
With this kind of configuration in camping site I have to remove and take out the extra seat for preparing the lower bed, otherwise it's impossible to sleep down.
Instead when we go around just for the weekend or 2/3 days without go in a camping site or when we have not the possibility to install the tent and put inside to it the extra seat, I leave the mattress and net at home, and for sleeping I use just the bench (with one kid) and put my legs on the extra seat closed. the rest of family sleep in the upper part of hotel.
For having more space between rear seat and bench, like you said, it is necessary put more back the bench, when I did this, I fixed the net with the mattress, directly on the back of the bench, using like anchorage/fixing points the headrests on the bench. In this position it was stable and I had n't problems during the driving.
To be honest, when i did so, I thought why the VW didn't tought an original support or something, for keeping the net and mattress in vertical position with the single extra seat installed.
Unfortunately I have not pictures available now about my solution, but hoping to be useful, I'm sure you have understood what I have done.
Bye