Beginning/Firstborn (18b): Paul opens this section by calling Christ "the beginning." Given His creatorhood (1:16) and pre-existence (1:17), one expects this title. However, Paul is no longer speaking of the created order, but of redemption, Christ's victory over death. He is the beginning as "the firstborn [pre-eminent one (cf. 1:15)] from the dead." Others physically rose from the dead, but none of them rose unto glory like Christ. When Christ rose in glory, He took "the first place in everything," even as the victor over death.
Fullness (19): Paul gives the reason for Christ's primacy: "For it was the Father's good pleasure for all the fullness to dwell in Him." The Father wanted His incarnate Son to have all of His fullness. His beloved Son must be fully supreme, whether authority (Matt. 28:18) or people (John 17:2) or judgment (John 5:22) or the nations (Ps. 2:8) or name (Phil. 2:9) or seat (Eph. 1:20-21).
Reconciliation (20): The Father's second good pleasure is the reconciliation of all things through Christ's sacrifice: "the blood of His cross." The Father made His Son the only path to peace with Him (John 14:6). Now, this peace is clearly with redeemable human beings (1:21-29), hence, Paul first speaks of those on the earth, and then mentions those already in heaven. Either way, God desired to make that peace only possible through His incarnate Son.
 "And He is the beginning" is the second relative clause "who is the beginning" a la 1:15 "who is the image."
 Jesus affirms: "I am the Alpha and the Omega, the first and the last, the beginning and the end" (Rev. 22:13).
 As it was in 1:15, "the firstborn from the dead" defines the meaning of "the beginning" as an apposition.
 No one was raised unto glory before He rose in glory. Only after Him did anyone rise thus (Matt. 27:52-53).
 "All the fullness" is God's fullness (2:9), hence, "all his fullness" (NET) and "all the fullness of God" (ESV).
 In this context of redemption of people, "all things [not in Greek]" is better taken as "all peoples/nations."