The "Hark" 3rd verse is printed as an alternate in "Hymns and Carols of Christmas," no source indicated. The 1st four lines are actually verse 5 of Sandys, and the last are his verse 4 in changed order.
It is part of someone's revision of William Sandys, "Hark! The Herald Angels Sing, For Christmas Day," 1833, "Christmas Carols Ancient and Modern."
Here are his five verses:

Lyr. Add: HARK! THE HERALD ANGELS SING (Sandys)
William Sandys, 1833

1. Hark! the Herald Angels sing,
Glory to the new-born King,
Peace on earth and mercy mild,
God and sinner reconcil'd.

Chorus (after each verse):
Hark! the Herald Angels sing,
Glory to the new-born King.

2. Joyful all ye nations rise,
Join the triumph of the skies,
With the angelic hosts proclaim,
Christ is born in Bethlehem.

3. Christ by highest Heaven ador'd,
Christ the everlasting Lord!
Late in time behold him come,
Offspring of a Virgin's womb.

4. Hail the heaven-born Prince of Peace!
Hail the *Sun of righteousness!
Light and life to all he brings,
Risen with healing in his wings.

5. Mild he lays his glory by,
Born that man no more may die,
Born to raise the sons of earth,
Born to give them second birth.

*Sometimes mis-printed as 'Son,' which was used in Wesley's original.

In the DT, the verses are combined into three, eliminating a couple of choruses, and, in a peculiar second verse, is included a verse found in a broadside from Cornwall, but not used by Sandys.

Veil'd in Flesh the Godhead *He,
Hail th' incarnate Deity;
Pleas'd as man with man **appear,
Jesus our Immanuel here.
* not see! ** replaced with 'to dwell,' an option used in the DT.