This is a wish or blessing. John speaks as if these were things that God could give, although they are really ways in which he hopes God will act for his people. Alternate translation: "May he who is ... and the seven spirits ... treat you kindly and enable you to live peacefully and securely" (See: [[rc://en/ta/man/translate/figs-abstractnouns]])
From among all those who have died. This expression describes all dead people together in the underworld. To come back from among them speaks of becoming alive again.