“But the first premise — that the faithful departed are dead — is false, and directly contrary to Scripture.”
Unless you are Jesus Christ, you don’t know who is living in heaven and who is not, since we don’t have the ability to see into peoples’ hearts and judge how faithful they were. Since none of us here are Jesus Christ, we cannot assume someone who has passed on is actually alive in heaven. We can hope, but we can’t assume.
In Matthew 16:19 Jesus gives authority over his Church to Peter, to bind and loose: "Whatever you bind on Earth shall be bound in heaven; and whatever you loose on Earth shall be loosed in heaven."
In Matthew 18:18, he gives the power to all the apostles: "Amen, I say to you, whatever you bind on Earth shall be bound in heaven, and whatever you loose on Earth shall be loosed in heaven."
The Apostles passed this authority on, down through the millennia to their successors through the laying on of hands.
That's not true in the case of canonized saints. Their presence in heaven is frequently confirmed by miracles: and that would be God's own testimony.