The difference is that a paper wallet is a separate wallet that you manage manually, whereas getprivkey exports one key (of the many) that is already in use by your online wallet. Printing this key does not provide any extra security against hacking, and there are no guarantees that the funds will stay on that key when Bitcoin-Qt has access to it.

It is best to generate the private keys and public addresses for a paper wallet with a separate tool on an offline computer. After doing that you can send coins from your online wallet to the offline wallet.

When you want to spend from a key in your offline wallet you can import the key into your online wallet and scrap it from the offline wallet (as it "has seen the internet"). Any change you must send back to a *new* address in your offline wallet.

It's an advanced debug feature. It's not recommended for casual use.
Not recommended. There are lots of keys in the Bitcoin-Qt wallet, and backing them up one by one in this way is going to be very error-prone, especially as there are change addresses that you can't see.

To backup your online wallet you should use the "Backup wallet" menu option, or copy your wallet.dat when Bitcoin-Qt is not running.  Save to some external location (such as an USB stick).

A feature to export the wallet in a human-readable format is underway.

Using import is okay as long as you know what you're doing. For example, you should be careful to not use the address as offline wallet address anymore after you imported it into an online wallet. One key must only be in one wallet at a time.

So you should never import a key that someone that you don't fully trust sends to you. If the same key is in multiple wallets, bad stuff can happen (wrong balances etc).

You got it.

Be sure to always have minimum 2 copy of your cold wallet EVER.