I don't think that's fisible. By just copying your wallet file the hacker would still need to input your seedphrase or keys to access your wallet. So always make sure to store your keys offline and not in your computer even if it's a cold storage..
This is wrong.
Anyone who has access to your wallet file (and knows the password, if it's encrypted) can steal your fund without any need to your seed phrase.
Take note that the wallet file contains the master private key and that can generate all the private keys.
Generally, for accessing your fund, you need to have either the wallet file (+ password, if it's encrypted) or the seed phrase (+ passphrase, if there's any).
You mean unlike BIP39 master private keys, on Electrum master private keys are reversible? That is to say they allow to calculate the Electrum seed? Because the seed is usually available in Electrum software wallets, so if the wallet file only cointains the master private key actually it means the wallet is able to calculate the mnemonic seed from this key.