What about, why do you think the elder calls Him the Root and Offspring of David in ch. 5, and in chapter 22 He calls Himself the Root of David, leaving off the offspring description?
Since Christ is called "the Lion
of the tribe of Judah" (Rev 5:5) it follows that He is also the descendant or "offspring of David" (who was from that tribe), and it was the tribe which would produce the kings of Israel according to Jacob's prophecy in Genesis 49 (see below), leading to King/Messiah.
Is there any significance to pairing the Lion with the Root and Offspring, and pairing the Morning Star with the Root?
Yes. Since the lion is regarded as the king of the beasts, the lion metaphor is used in this connection with Christ as King/Messiah from the tribe of Judah.
Judah is a lion's whelp: from the prey, my son, thou art gone up: he stooped down, he couched as a lion, and as an old lion; who shall rouse him up? The sceptre shall not depart from Judah, nor a lawgiver from between his feet, until Shiloh [Messiah] come; and unto Him shall the gathering of the people be. (Gen 49:9,10)
The "Morning Star" is a reference to the Christ who is the Light of the world, as well as the light of the Gospel.
Nevertheless the dimness shall not be such as was in her vexation, when at the first he lightly afflicted the land of Zebulun and the land of Naphtali, and afterward did more grievously afflict her by the way of the sea, beyond Jordan, in Galilee of the nations. The people that walked in darkness have seen a great Light: they that dwell in the land of the shadow of death, upon them hath the Light shined. (Isa 9:1,2) [Note: this is quoted in the Gospel of Matthew 4:13-16]
As to the "Root of David" it refers to the fact that God brought the tribe of Judah into existence when Christ blessed Jacob and called him "Israel" (a prince with God and men). Then God chose David out of all the sons to Jesse to be king over Israel. Thus David has His *root* or origin in Christ, yet Christ is also His offspring. This anomaly is addressed in Matthew 22.
MATTHEW 22
41 While the Pharisees were gathered together, Jesus asked them,
42 Saying, What think ye of Christ? whose son is he? They say unto him,The Son of David.
43 He saith unto them, How then doth David in spirit call him Lord, saying,
44 The LORD said unto my Lord, Sit thou on my right hand, till I make thine enemies thy footstool?
45 If David then call him Lord, how is he his son?
46 And no man was able to answer him a word, neither durst any manfrom that day forth ask him any more questions.