First is appears that the action between the steel bolts and the aluminum head formed an extremely tight bond. That is why the bolts needed to be removed. The head gasket failed and glycol anti freeze both liquid and dried were all over once thr head was removed. Exhaust gas probably helped cook the glycol glycol . Three dissimilar metals copper, aluminum and iron did not help. Peeling the head the head was possible due to bolt removal. It is my opinion that with such a strong head/block bond, attempting to remove using thread plugs in the Aluminum spark plug holes would have stripped them out.