Hi Xender.
I can’t speak for Just Flight, but in practice it wasn’t possible to walk through the cabin when in the freight mode since there was always a freight net positioned just aft of the fwd vestibule preventing easy access aft. It could be moved aside (not easy), and while you could always look, you didn’t enter as to get down the side of the containers you would have to be very small indeed.
When carrying horse boxes, pax seats would be positioned aft of the boxes for the grooms and veterinarians.
Lots of extra fire containment and warning, plus the large rear freight door and the tail support required (NEVER forget the tail support if you want a quiet life!).