'nojwod' there would have been some mixing, Jet & Avgas does mix otherwise turbines engines wouldn't be allowed to use Avgas (limited use) for engine combustion.
You might find that in yr dads case the car initially ran on the fuel in the fuel lines & carby (if it where a carby fed donk which I think the old 504 was) before the kero based fuel came into contact with the engines combustion chamber/s. When such an event happens the damage to the engine is nasty thru sever detonation problems causing failure of some moving parts within hence the 'dead stop'. The large cloud of smoke as you mention was the final explosion of the poor fuel/air mixture just prior to death of the 'Pug'.ugly things they where too
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The biggest diff between Avgas (petrol) & Avtur (Kero) is the liquids flash point not so much it's ability to mix. As most would know for an auto eng you need a much higher compression to support ignition using Diesel fuel where using petrol (Avgas) it can explode just looking at it!
Wmk2