No, by definition, there's a big difference. You have to be justified to be saved. Justification is an entirely free gift.
Yes, but that doesn't mean there's no difference between justification and salvation.
It's only optional and unneeded insofar as being transferred from being unrighteous before God to being righteous before God is concerned. The man in Luke 18:13-14 did nothing except ask for God's mercy to be declared righteous.
Just because this is what some people think, that doesn't mean justification and salvation aren't different terms for different things. You don't have to think you can live in unrighteousness after being saved to believe that justification and salvation are different things.
A man is justified - that is, made righteous in God's sight - entirely as a free gift of God's grace given through faith in the Son, apart from the merit of his righteous deeds. Just as that was true for our example, Abraham, who was made righteous by believing God's promise about a Son apart from the merit of his righteous deeds.
Righteous deeds follow being made righteous by God and are a direct result of being made righteous by God, and understandably so. For it is from the new nature of righteousness that we have received that we then live for God in righteousness. And so the obedience of the holy life is the sign and evidence that one is justified in Christ. And without that evidence -the evidence of saving transformation - no one will enter into the kingdom of God at the end of the age when Christ sorts the righteous from the unrighteous.