It doesn't, because you can use a piece of software to calculate the checksum for you. You must use a piece of software to turn your seed phrase in to private keys and addresses - doing this by hand is simply not feasible. So requiring that same piece of software to also calculate your checksum for you brings zero additional risk.

I think that proves my point perfectly. And the exact same character swap (g -> q) is easy to spot in the seed phrase in the word "ceilinq". Additionally, you can spot an error in a seed phrase like this without having the original to compare to. With an address, if you have no original to compare to then you can't spot anything at all.

If your plan is to randomly guess the seed phrase, then you don't need to steal the hardware wallet first. Just start generating seed phrases and checking for a balance, but note that the Earth will become uninhabitable by the dying sun expanding to a red giant long before you find a collision with any wallet, let alone a specific one.

Repeats are allowed. Provided your seed phrase was generated properly, then there is around a 1 in 31 chance of a repeated word in a 12 word seed phrase, and a 1 in 8 chance in a 24 word seed phrase.