My vote on this is that you avoid 2FA requiring third party assistance to move coins. Not knowing what the future holds I believe its better to maintain 100% self custody of your coins.
Not a big fan of TrustedCoin but that isn't true. You're still maintaining 100% custody of your coins with 2FA since it is a 2-of-3 multisig and you hold 2 of the keys while they hold a single key. They cannot do anything without your approval but you can spend the coins as and when you wish, provided that you have access to your seeds. I'd argue that 2FA provides a marginal increase in security and I agree that an airgap setup would be vastly more secure than 2FA.
But now somebody can brute-force the extended words of the seed phrase. True, the existing bitcoin wallet crackers such as hashcat and btcrecover, do not support this kind of recovery with seed phrase input at this time, but a) the seed phrase can always be sold on the darknet, and b) it could end up with someone who has a custom tool for solving this kind of stuff.
The last thing you want to happen to you is having your seed phrase end up on Google Search. It is NOT safe to continue using it, not even with additional password or 2FA. I recommend moving all your funds out immediately before they get stolen.
Your seed is designed to allow the user to access the coins with the seed only in the case of 2FA.
It depends on the passphrase. If your passphrase is long and random enough, there is very little chance someone would ever be able to be able to bruteforce it. I don't think Electrum limits the length of the passphrase. If it is long enough, then it would be equivalent to be bruteforcing without any prior information.