I think Christ's sermon in 3 Nephi 20 is best understood when we realized that he is delivering it in a temple context, in the middle of a temple ordinance, to temple initiates. For instance:
The remnant of the House of Jacob are a covenant people. Which covenant? The covenant the participants have all just made, the covenant of the "prophets" and of Abraham (administered to Abraham by Melchizedek).
The "powers of heaven" will be with them. Christ himself will be with them. Christ is the power of heaven, the Lord Yahweh who dwells behind the veil and who has come forth to give them his flesh and blood.
When the "fulness of the gospel" (temple rites and doctrine) is preached to a people, then can their "watchmen" (= angels = priests) sing "together," because of their vision (they "see eye to eye").