I have a question regarding the second command.
The:
dumpprivkey "YOU_ADDRESS"
where can I find the address I put there? Is it the public address of my wallet?
I do not own the public address anymore, I just have the wallet.dat + passphrase. Will it be enough to enter the "new" pub key to the dumpkey command?
You first need to find out which addresses inside of your wallet contain a balance.
You can check this by entering them in a block explorer (e.g.
https://live.blockcypher.com/).
If you don't have a clue, you should either just let it sync or dump the whole wallet using this command:
dumpwallet "filename"
Then simply import all private keys into electrum.