Yes, on many of the van models, the front seats are on removable bases, allowing the wheelchair passenger to ride up front or drive.

The transit is better suited for the (much cheaper) wheelchair lifts, instead of the integrated ramps. RWD makes a ramp conversion very difficult and the larger van has enough space to fit a full size lift, much like the old E series/Express

Correct, the Explorer is FWD only

The testing is outsourced, but the vans are designed to be safe to drive with the wheelchair restraints.

The vans are definitely crash tested, in different configurations. There are NHTSA regulations for accessible vehicles just like for OEMs. As far as seating goes, removable front seats are available on some models, which allows the person on the wheelchair to ride up front or drive.