this is why i don't think its det without looking at the head phyically, with the plugs being all the same colour it suggests that cyl 1 wasn't running that hot.
Also for det to damage the engine that much i think you would most certainly hear the engine pinking its arse off, i had bad det on my lader set up and ran that for about 1k miles with it detting its arse off and the head and piston showed no damage whatsoever when i changed the head gasket.
i am still wondering if something has gone through your intake system, but having said that i have just look at the picture of the head and that big hole at the bottom looks like it has melted!!!!