Will someone have access to your wallet without your device but the person manage to see your private keys and copy them without your knowledge. Will the person be able to withdraw your money or change password and block you from having access to your wallet?
They won't be needing to know what your password is. They can set up a new one once they entered the seed phrase to the system. If they can do it, that means you can do it too.
Also, there is no way that they can OR you can block each other from accessing the wallet WHILE you both knew the seed phrase exactly. As far as I know, that feature is not existing.
It's best to not store your seeds online, take the hassle and write it down then keep it to the safe place.