When someone dies in the state of grace, it is true that that soul is guaranteed heaven, but it does not mean that the soul will go directly to heaven, since there may still be need for purification before a soul can see God face to face. This is what we call purgation, or the purgatory.
This is so because every sin has two Effects: it harms our relationship with God, and it affects others. Being in the state of grace heals the damaged relationship with God. The damage it caused on others can only be healed through reparation in this life and purgation in the next if not yet completed in this lifetime. The second effect however can be supplied through the Church. This is what indulgence is for. Plenary indulgence removes all outstanding temporal punishment from a soul.
And so, when someone passes on, even when there is good reason to believe the person was reconciled with God, for a Christian there is always both a joyful hope that this soul will gain heaven, and at the same time, a sombre awareness of the need to intercede for this soul’s final purgation, for a soul in purgatory cannot help himself, except through our prayer and act of reparation. This is why we have the tradition of offering masses for the dead, and the dedication of the month of November to pray for holy souls in purgatory, that they may gain heaven soon.
As for your second question on the Final Judgement. When we die, there is immediately a first judgement, the Personal Judgement, in which we will be shown our whole life how we have responded to God’s many acts of mercy, and understood fully what we have freely chosen all our life. God therefore does not “judge” us per se, but gives us what we have always wanted by our own choosing. Those who has chosen evil to the end freely go to hell. Those who have chosen mercy during their lifetime tend toward God. But not all are yet perfect, and so some of these need to first go through purgation. Yet all who enter purgatory, eventually go to heaven.
At the second judgement, the General Judgement or Final judgement, our destination of hell or heaven will not change. But at the Final Judgement, the world as we know it has come to its final consummation. Christ the King has returned, Satan and his minions forever vanquished, and all the dead have their bodies resurrected: both those in hell and heaven. So the bodies of those in hell will share the suffering, and the bodies of those in heaven will share the glory. They will be brought before God and this time see not just each his own life, but all will see all God’s works, and how everyone responded to His mercy. All acts of all persons will be revealed as in broad daylight, so that all will praise GOd’s mercy in the salvation of those who accepted mercy, and praise His justice in the damnation of those who refused mercy. Heaven and earth will then be remade and all in heaven will inherit His glory forever.
But as St. John said: What we will be like we do not know. BUt we know that we will be like Him, for we shall see Him as He is (1 Jn 3:2).