Saying that they all ended up against the rear bulkhead is silly
Why so?? If the C of G was marginal with the parachutists in their normal T/O positions as quoted in the interim accident report, then surely the sudden pitch up on rotate would very likely have caused the front two pairs, against the fwd bulkhead, and not secured in any way to the aircraft, to pitch aft, they would then have ended up amongst the pair seated at the rear bulkhead, this would surely have put the C of G so far aft as to make the aircraft uncontrollable. The distance the front pair could move aft in the cabin I am not aware of, but from the photo's of the accident aircraft cabin it would amount to a number of feet.
This scenario very closely parallels what actually was observed from the ground.
Superair tested this and found it was easily controllable in the Walter
That may be so, but the test pilot knew what to expect, if this problem is thrust upon you with just a few seconds to assess and the rectify the problem I would say that info is of little use. The load shift would have happened in a second or two, on rotate, as observed by witnesses, and with the attitude of the aircraft no way to clamber back to your proper station.
It was normal practice
for the first 2 tandem pairs to sit as far forward as possible facing rearwards. The next 2 pairs
would then sit with one pair opposite the door and the third pair against the rear bulkhead
facing towards the door
If it was possible for the investigators to establish that the manual trim was in the T/O position, then surely it would have been possible to establish whether the control column had broken, something I have never heard of happening in the many thousands of hours topdressing in FU24's