The design of the Scout heater and HP cock was not so much ridiculous as the heater is a lateral slider wheras the HP cock is a fore and aft gated lever. As a Scout QHI I made it a serious teaching point not to mistake the two and a variety of methods were used to ensure, as far as possible, that they were not confused. When flying two crew for instance, he who would operate the heater would put his hand on it and say "heater", the other crewmember would look in and either confirm or scream "no you dumbass thats the HP cock"!!
I can remember at least 3 occasions during my 2000hrs on Scouts where I screamed the latter. It proved the CRM worked.
My point therefore is that no matter how good or bad a design may be perceived, human error and poor cockpit drills are normally to blame for mishaps like this.
As far as civvies flying Scouts is concerned it was only a matter of time until a mishap occurred, I can tell you all for sure that the only reason both of the civs survived is as a direct result of the quality training they did receive prior to flying solo.