I once worked at an organisation where the engineer was not particularly interested in what he was doing and wouldn't fly in an aircraft. Lots of things didn't get done properly. It got to the point where people would refuse to fly aircraft if they had just come out of the engineers workshop. Luckily, he was sacked. Personally I think all engineers should have to have a PPL and should have to go and do a flight test before releasing any aircraft to service. I wonder if this engineer would have gone spinning, knowing what he did.
I know that sometimes we all try to cut corners, but this sounds rather more obviously a corner which could not be cut.
I also always wear a parachute when doing aeros and would want to be recovered by a lot higher than 2000', not that it would have helped in this accident. I think it should be mandatory to wear chutes when doing aeros in any aircraft. That way, they would have had a chance at least. Its terrible for the instructor, he must have realised that the rudder was jammed and there was no chance of recovery. Still, its something that the student survived. If they had gone into the ground I suspect the result would have been even more tragic.