That weight on the bottom below the float is the most critical weight. It buffers out all the movement down the rope from wind, waves and current and leaves the traps sitting still. If your first trap catches less than the others, it's probably because it's moving around. Prawns will be reluctant to enter.
I like to have the boat moving forward gently as I set. Dropping straight down might cause that trap flip you're worried about. Bump it in and out of gear to maintain slight forward way and the traps and weights will spread nicely across the bottom. Do it moving upslope and you'll have traps fishing slightly different depths. Note carefully which produced, and set along that contour at best depth on the next set.