Your understanding is flawed. When you hash a block header that hash is ONLY useful for that exact block header. Period.
Merge mining doesn't swap data. If you swap data (and bit of data in BTC header) then it needs to be hashed again as the hash of the block header w/ different data has a different hash.
So how can one hash be potentially useful for both BTC and NMC?
The NMC protocol has been modified so that NMC data is ADDED (not swapped) into the BTC block header (in the coinbase transaction to be specific). Since BTC network doesn't use coinbase transaction for any purpose other than extra-nonce (adding entropy) there is no conflict.
Every "merged" block going forward has additional NMC data inside the header. Thus the hash of that header is potentially useful for securing the NMC block chain. You can look it up in the block explorer right now.
So the process is:
1) miner/pool adds NMC data into coinbase transaction
2) BTC merkle-tree root is computed as normal
3) miners hash trillions of hashes to find one with difficulty below target
4a) if hash is below BTC target it is valid for BTC network
4b) if hash is below NMC target it is valid for NMC network.
The NMC network can verify the hash is valid for BTC header which contains BTC merkle-tree.
The NMC network can then verify the BTC merkle-tree is valid for the coinbase transaction containing required NMC data.
Thus security for NMC network can be indirectly confirmed via BTC hash.
You can't accidentally hash multiple block chains. You or your pool must explicitly include EXTRA alt-coin specific data in the BTC block before hashing it.
Once all the major pools do merged mining for 20+ block chains, soon the last noob will understand that the hashing power behind any such block chain does not imply a USD/coin-rate lik many do here all the time. cosbycoin and poocoin will have 15TH.
It is highly unlikely 15TH+ worth of pool operators are going to add 20+ different block chain specific data into every BTC header. Those coins have little or no value. Namecoin is somewhat unique because it is established, has some value, provides a useful non-competing function, and has decent liquidity. Attribues all "scamcoins" lack. This isn't to say BTC block chain won't be used to secure other assets in the future but your scenario is based on assumptions that don't exist.
Lastly merged mining only works if the hashing algorithms are the same. Many alt coins intentionally use completely incompatible hashing algorithms. There is no method to merge mine those chains.
Dealing with a fork: With merged mining I can just mine for both and if I find a block, announce it for both chains, just in case.
I think you can see this is impossible now. If BTC chain gets forked to mine both forks requires hashing both independent headers thus requires 2x the hashing power (just like it did prior to merged mining).