Hello

The actual problem with the engine the head had gone soft and the gasket had eaten in to the head on the exhaust side, This would have happend over a long period of time with also left the gasket in poor condition, This was easily seen upon looking at the gasket and head,
Nobody was to blame the only way you would have seen this was to remove the head and inspect it,
Every head I work on is always hardness tested even if it is new, I have only seen this type of fault on 3 ocassions.

Mark