I had a similar problem recently. I tried everything and like you the engine eventually seized. The problem was a leak in the exhaust elbow which was spraying water back into the manifold internally. The exhaust manifold was completely blocked. I had to have the engine rebuilt even after cleaning the manifold because of some surface rust in the first cylinder which is why it was seized.
--
Bob Thomson
SY Memo
2001 Amel SM2000
Ser No 326