From what you say I think the cylinder head could be cracked or distorted. If the flange housing had nothing but coolant flowing through it all of the time it wouldn't melt because it could not get any hotter than the coolant (less than boiling point).
Considering the history, localised overheating invariably leads to distortion and often a crack as it cools.

I suggest you use a block tester with indicator fluid that changes colour when it contacts combustion gas. From what you say I think the cylinder head could be cracked or distorted. If the flange housing had nothing but coolant flowing through it all of the time it wouldn't melt because it could not get any hotter than the coolant (less than boiling point). Considering the history, localised overheating invariably leads to distortion and often a crack as it cools. I suggest you use a block tester with indicator fluid that changes colour when it contacts combustion gas.