I don't see why the number generated would be more likely to be cracked when the seed phrase has the same chance of being guessed.
Because the number is being generated by different processes using different entropy sources. If your entropy source is poor, then your number won't be completely random.
If I put the seed phrase in any other wallet, it will generate the exact same addresses in the same order with the same private keys.
This is a completely separate (and completely trivial) function to generating the seed phrase in the first place.
I don't understand why you are arguing about this, especially when it is clear you do not understand the basics. Javascript key generators are insecure. This is a fact. There are plenty of easy to use alternatives already suggested. Electrum, for example, is already bundled with Tails, so is trivial to use since you are planning to use Tails anyway. As I said above, you seem very keen to use an insecure javascript generator and there is nothing we can do to stop you, other than warn you of the risk.