"The God of Peace" is a Pauline phrase. Look at Romans 15:33 and the parallels. He is the God of peace and of grace, not a sovereign God. If God could bring back Jesus from the dead He is able to do what is prayed for in verse 21. Look at John 10. AAT: ". . . Jesus, who by His blood made an everlasting covenant to become the great Shepherd of the sheep." Only here in the New Testament is the covenant called "eternal."
Theodoret: He called the New Covenant eternal because after this one there would be none other.
Guthrie: There is no possibility of its becoming obsolete and another being needed.
Kistemaker: The covenant is everlasting because it is sealed in blood -- to be precise, the blood of the Messiah.
Kretzmann: He shed His blood in consequence of God's covenant of mercy.
Lenski: Why not think of Jesus'own word 'my blood of the new testament' (Matthew 26:28) and of all that this epistle has to say about this very testament and about the blood by which it is established?
Kistemaker: Through the prophets Isaiah, Jeremiah, and Ezekiel, God announces his intention to establish an everlasting covenant with his people (Isaiah 55:3; 61:8; Jeremiah 32:40; 50:5; Ezekiel 16:60; 37:26).
The words "our Lord Jesus" are placed last for the sake of emphasis.
Verse 20 is the complete subject. Verse 21 is the complete predicate. "May he work" is an aorist optative of wish. The word "artist" is derived from this word. "In" would be translated: "In the sphere of every good thing." God alone can cause man to do His will.
Westcott: Each deed is at once the deed of man and the deed of God. The work of God makes man's work possible.
The concurrence of God and the believer is truly a great mystery. This is not synergism. Synergism is the false teaching which attributes the origin of spiritual powers to sinful human nature. It is always "through Jesus Christ," Who said: "Without Me ye can no nothing." First God must "fit us out" and then continue doing in us. We are to do, and yet it is God at work in us. To do God's will should not be restricted to doing what are called good works; it includes, above all, faith in Christ, John 6:40. Very likely "what is pleasing" is adjectival: "That which is pleasing in His sight." Likely the antecedent of "His" is Christ.
Westcott: The doxology may be addressed to Christ as in 2 Timothy 4:18; 2 Peter 3:18; Revelation 1:6. But it mostly likely refers to the main subject.
Some say it refers to the persons in the Godhead. It is not a problem.