Hey, my friend... long time (again).
I'm sorry to know that you lost the key.

The thing I know about recovering an account is that you need an old master password at least and then you'll request for recovery and you'll get a link in your email associated with that account and with that you'll get the new key... that's what I followed long before when someone stole my password. IF you don't have an old password as well then I'm not sure if you'll be able to recover it but there should be a way to recover the account from the account that was used as a recovery account and in this way, your id must have been associated with an e-mail.

That's what I know about this... I might be wrong... :-)

Delegate them to my other account

Did you check it with the posting key? I thought delegating needs the activation key as well.

But obviously he can delegate the cards with this key to other accounts.

Yeah, that's possible.