my experience as a Jump Pilot, which includes a fair amount of years dating back to the mid 80's to 2001, all as a non payed pilot, was that all behaved very well once in the aircraft. Most of the 'playing up' or 'bravado' as one has mentioned, was on the ground, but once inside the aircraft all behaved appropriately. The latter years were all dealing with Tandems and the solo's were generally for filming the punter that had paid for it. Not once did I feel that these individuals, no matter which outfit I flew for, behaved in other than a professional, business (but fun) like manner. I have not flown the fletcher but C172, C206, C180, and C185 in PJE ops and at all times they knew where to sit, where to stay, where their fare paying punter was to sit - and that's where they stayed until it was time to pin up - all without having to be told by me.
Runaway trim is feasible - its nasty and there is nothing you can do (unless you recognise it and have time to pull the circuit breaker) - my experience - just airborne in a PA18-180 towing a glider I pushed nose down on the electric trim and it kept going. Had the glider not released when I yelled on the RT to 'release', I might not well be here.
Another 'theory' above is control lock. Again not familiar with the fletcher - Pilot of a Cub in TG a few years ago had the seatbelt around the pax stick and took off with a glider in tow - got airborne and went into the vertical. Seatbelt was used as a 'gust lock' - would appear pilot forgot DVA's prior to takeoff. A/C stalled and pilot killed.