Is this a once for all type of event? When the person says the "sinner's prayer" are his future sins forgiven preemptively?
What you refer to as Justification is a juridical understanding of what can be easily described as our rebirth into the Family of God... our redemption. Your description of Sanctification is another juridical understanding of simply being an active member of God's Family. I thought Protestants said Catholics made things complicated...
King David was an adulterer and murderer but God forgave him and considered him righteous.
He was forgiven and restored as a result of his confession and penance through God's Grace. Now... who follows this model today?
I don’t take stock, necessarily in the sinner’s prayer. Whenever a person comes to the point when they truly believe in their heart and have repented of their sins, then they are adopted into the family of God and positionally declared righteous before God. That could happen during someone saying the sinner’s prayer if that person truly understood what it means to accept Jesus and repent of their sins. From then on, they are part of God’s family, and as Jesus said, no one can snatch them out of His hand.
If a Christian does sin, then they may face earthly consequences, which is God’s way of disciplining them into conforming to His image. That is not to say, though, that each time they sin they have “lost” their redemption and can only reclaim it by confessing that new sin. Their position in Christ is secure because the penalty for their sin was paid for by Christ.