However, if my understanding is correct, in my thought experiment it is easily possible to generate the private keys and addresses that would be created by a real Electrum wallet using the 12 words.
If someone knows your 12 words, it is trivial for them to generate your private keys and addresses.
If someone does not know your 12 words, it is impossible for them to generate your private keys and addresses.
While large amounts of computing power might be needed, this attack would work against airgapped wallets as well as those on-line.
Even if you used every piece of computing hardware in the world for this task, and consumed every single joule of electricity in the world to run it all, you would not find a collision.
Is this a feasible (if computationally expensive) attack
No.