I think you still didn't look at all into it and have actually no idea what you are talking about. I'm sorry to be so blunt but it's gettin a bit annoying
What do you mean by keyfile ?
You have 1 or more wallets. 1 wallet can hold many accounts but that doesn't even matter right now.
Your wallet is an encrypted file stored locally on your computer that hold your key.
Noone can decrypt it without the password you chose and as long as you don't pick a crappy password.
Noone will be able to guess your key unless he is the luckies son of a bitch in the entire universe.
Bitcoin uses the same system. Ever heard of a wallet being compromised by guessing a key ?