1. I will try to help, for dump wallet, I do not really know about that, so maybe someone will explain more about this. I found a good post about this, may be can help you to understand.
To export all the keys, you have to use dumpwallet command. The command will export all your keys and your master private key which can generate any future address
unencrypted so use it with care.
1.Go to help>debug window>console.
2. Key in this if you have your wallet encrypted:
walletpassphrase PASSWORDHERE 600
replacing the relevant parts.
3. Create a blank txt file in whatever directory you want and name it walletdump.txt. For example, I stored mine in C:/ with the name walletdump.txt.
4. Key in this:
dumpwallet C:/walletdump.txt
. The command will export every of your private keys into that text file. You can change the directory if you stored your text file on another directory.
But, if you just need 1 private key, you can use command
dumpprivkey and follow with your Bitcoin address. You can follow step 1 - 3 above, and then you type command
dumpprivkey YOURWALLETADDRESS
2. Yes, backup wallet is mean you need to copy your wallet.dat file to another location. But make sure if you are creating password to open this wallet, you must remember that password too.
3. You don't need to do all of that way. If you already have a seed passphrase, you can safe this one only. But, if you have a double backup file of the wallet, it will help if you lost one of them.