Another use of "sealed" is in order to confirm or attest to something - to authenticate. Christians do that through their baptism. It is our agreement to God's terms of the New Covenant. We CAN violate that agreement and break the covenant. We fortunately have the option of confession and repentance. Those, I think, who fail to do so exist outside that covenant.
I have looked at your statement regarding (only?) those who alive go before the Great White Throne for judgement. I don't find anywhere that suggests that, so perhaps you can show me. What I do find is:
Jn 12:46–48 I have come into the world as light, so that whoever believes in me may not remain in darkness. If anyone hears my words and does not keep them, I do not judge him; for I did not come to judge the world but to save the world. The one who rejects me and does not receive my words has a judge; the word that I have spoken will judge him on the last day.
Ac 17:30–31 The times of ignorance God overlooked, but now he commands all people everywhere to repent, because he has fixed a day on which he will judge the world in righteousness by a man whom he has appointed; and of this he has given assurance to all by raising him from the dead.”
Re 20:11–15 Then I saw a great white throne and him who was seated on it. From his presence earth and sky fled away, and no place was found for them. And I saw the dead, great and small, standing before the throne, and books were opened. Then another book was opened, which is the book of life. And the dead were judged by what was written in the books, according to what they had done. And the sea gave up the dead who were in it, Death and Hades gave up the dead who were in them, and they were judged, each one of them, according to what they had done. Then Death and Hades were thrown into the lake of fire. This is the second death, the lake of fire. And if anyone’s name was not found written in the book of life, he was thrown into the lake of fire.