Bongo Fury

Can a middle seat from one van be used to replace a missing rear one in another ?

I can't see why not. I fancy having the same myself, it would be idea for fishing rods etc.
Plus easier for kids to claim into their seats, from the back door.

yes definitely. I am about to use my middle bench seats in the back of my bongo.

Only with long runners?

I've just fitted long runners & my reversed middle seats won't slide all the way back due to the mounting points for the split seats fouling on the plastic trim on the end of the seats . I haven't investigated further as to what "adjustments" may be possible to make it happen as yet.

mine will be done on long runners for two reasons. firstly i currently have split seats so I have to add the runners anyway rather tha use what is there and secondly because the floor can be made nicer afterwards with the long runners.