How does namecoin and other merge-mined coins prevent pre-mine attacks?
If you fake the bitcoin blocks, then you will be mining only namecoins and no bitcoins, so you are losing 99% of your mining income.

You can't just mine at a height above the current namecoin blockchain. Remember that it is a chain. Each block has to link to the previous one. If you want to mine at namecoin height X which is thousands of blocks above the current height, how will you know in advance what the hash of the block at height X-1 will be?