Regarding chances of having a locked “full up” elevator situation, my brother did experience just that. He was involved in a gaggle midair over Cal City during a regional. He was flying the then new to racing Ventus A model. His elevator was impacted by the wing of the other glider which resulted in having an elevator jamming in full up deflection. His ship did a series of three high speed loops before he was able to kick the canopy away and get out. He was under pretty high g’s as he told me he could barely move and had real limited eyesight. He was lucky to barely have had enough altitude for his chute to open. The ship now sans-pilot continued to loop itself right down to the ground. She landed herself in the sagebrush ending up with a broken tail boom, semi destroyed elevator and crumped control rod/linkages, and dented fuselage but no noticeable wing damage from the pilot-empty landing.

Regarding that duo discus accident, there is a definite mystery there, I do not think they found any control surface related issues in the investigation. but any way you slice it, be it two guys stunting, two guys fighting each other for control, one guy thinking the other guy has control and doing nothing, one guy having a medically related episode, etc, its another example of another needless accident.