I guess this topic fits better to the altcoin section. What particular problem do you see with merge-mined coins that have different block times? I see none. The prev hash points to the previous block hash of the chain in question (Huntercoin in your example), just as always. Why do you think it should be different?
Maybe I got confused about parent block and bitcoin block. I tried to understand, a parent block is not eqaul to a bitcoin block, In auxpow block, the previous parent block hash points to the last parent block not bitcoin block. I'm right ?