I think you may be over-thinking it. The “stuff” is no different from any other load. You wouldn’t suddenly re-insure if you took the seats out and loaded it with green waste to take to the recycling centre, or stacked boxes all over the seats to take one of the kids to uni. Minibuses are sold to be reconfigured and used like that, so the insurer knows what they are insuring.Insurance can get a bit complicated if you reconfigure a van / bus like that. What do you insure it as? A minibus with X number of seats? If so, are you then covered if you take some seats out and fill the vacant space with other kit that might or might not be secured in the event of a crash?
Where it might get tricky is if you start putting windows on a panel van, which may change its “class”, although I’m not sure what current legislation says.