Where does the offline passphrase play into this?
What "offline passphrase" are you talking about? You haven't explained what you're trying to do.
I assume you have a "paper wallet", and you're trying to figure out what to do with the private key when you want to use (spend or sell) the bitcoins, but I'm not sure.
As of now I don't have anything, but you are right. My questions is exactly what you stated. Do you need the private key to spend or send the BTC? Does the paper wallet private key have to go online in order to do this?