The brute force requirement gives you the economic security against the attack.
Sure, but there is no requirement for me to also have to brute force my own keys to provide security against an attacker brute forcing them. I could memorize a 12 word seed and a passphrase - I don't have to brute force anything to access my coins, but an attacker still would.
maybe relative easier but still unfeasible to guess. I bet 1 btc, you or anyone else wont be able to hack this.
Depends who the target is. If you were to know someone's name and could look them up on Facebook or other social media, you would have a much greater chance of hacking this than you would of hacking 12 random BIP39 words.