The questions is based upon a false premise that there are fallen heavenly messengers / angels. The false belief of "fallen angles" came about during the intertestamental period under pressure brought by the Selucid and Greek rulers that wanted Jerusalem and Judea to conform to pagan Greek social and religious practices.
There are no heavenly messengers / angels that have sinned, or fallen out of heaven. That is to presume that God's heavenly messengers were subject to the Law of Moses handed down at Sinai. It is also to presume that the heavenly messengers do not know and understand the power of our heavenly Father whom they see, and face every day! The heavenly messengers do the will of our Father in heaven (Psa. 103:20).
(See "Testing The Spirits Part IVa - Slandering Angels here and Part IVb here as well as the posts by Stan Lindsey cited therein.)
Further, the question also presumes, as is commonly taught, that the "prince of this world" (John 16:11), or the "god of this world" (2 Cor. 4:4) referred to the "devil" or "Satan". That false belief also comes from pagan theology of the Persian Zoroastrian belief in a two-god religion.
First audience perspective will reveal that there was a "god" of their world - the Roman empire, the 4th beast kingdom of Daniel - that ruled over their world - the Caesar who called himself a son of "god". That particular Caesar who ruled the Roman empire during the time that both the gospel of John and 2. Corinthians were written was Nero.
The phrase "the god of this world" never referred to the devil, or Satan, as our Father in heaven has never abdicated His rule and authority over this earth (Ex. 19:5; 1 Chron. 29:12; Psa. 47:2; 103:19; Psa. 22:27-28; etc.) See the post "The God of This Age..." ShreddingTheVeil.)
And further, the question assumes that the pronoun "we" in 1 Cor. 6:3 includes the audience. Who does "we" refer to but the apostles of whom Paul is one of the ministrants to the assemblies of Christ, the ecclesia.
The context of 1 Cor. 6 was of the brethren suing each other before unjust courts of unbelievers. Paul was telling them that they have discernment to judge within the assembly, and that the ultimate authority over them were the apostles to whom Christ had given the authority as ministrants to the assemblies. The Apostles would be judging "messengers / angels" of the gospel of Christ, those messengers who might be trying to alter or change the gospel message (1 Gal. 1:7-10).
(See also the post at my blog "The First Audience Perspective of Romans 13" here.)
Therefor, as there are no fallen angels, there are no heavenly messengers / angels awaiting judgment. And, as the "god of this world" was Caesar Nero who had already been judged by our heavenly Father, it does not apply to 1 Cor. 6. And, the judgment of the things of this life does not refer to judgment of things in heaven, the next life.
The promise of reigning with Christ in heaven does not imply that we will take on His role of judgment in heaven. All judgment is left to Christ (John 5:22, 27; 8:16; Rom. 2:16; 2 Tim. 4:1). No where do the scriptures say that we will judge in place of Him or for Him.
The answer to the question is that there are no heavenly messengers waiting for judgment. All messengers subject to judgment are earthly men who are unbelievers, or who have fallen away from the gospel of Christ.