The message of the Apostles and the Seventy (Luke 9, 10; Mathew 10) was “Repent for the kingdom of heaven is at hand.” This was a message for the repentance of that Nation consistent with the OT prophets (all the way to John the Baptists and the Messiah - Malachi chs. 3, 4).
National repentance was sought so that the Nation could once again be in fellowship with Jehovah, and that Nation be the Chief evangelical witness to the Gentile nations.
There is no message of the Death of Christ for the payment of sins in the Synoptics prior to the Resurrection of Christ, preached by the Apostles, as it is clear from Luke 18:31-34 - “ . . . they understood none of these things: and the saying was hid from them, neither knew they the things which were spoken.”
This is further enunciated in Matthew 16 itself, directly under the kingdom and keys reference, when Christ, “From that time forth BEGAN”(it was the first time He taught it) to teach them about His sufferings. “Peter took Him and began to REBUKE Him, saying, be it far from Thee, Lord: this thing shall not be unto Thee.” (Matthew 16:21-23).
They had already been out preaching a “gospel,” and therefore it must be considered that the “good news” they had been commissioned to preach in Matthew ch. 10, did not include any information about the sufferings of the Messiah. It was the “gospel” (”good news”) that the Messianic Kingdom promised through the Old Testament prophets was “at hand” and ready to be re-established on the earth under the Messiah.
The message of the Twelve and the Seventy in the Synoptics prior to Calvary had nothing at all to do with anything we see being carried out on Earth today. But it IS coming.
James 5:16 itself is post resurrection, and is itself written Twelve Tribes scattered abroad (1:1), and not to the Body of Christ. The Body of Christ is not divided in to twelve, or in to any number of tribes. The Church is not a tribal people as Israel was.
2 Corinthians 5, the Ministry of Reconciliation, was revealed first to the Apostle to the Gentiles, Paul, and was never a subject of teaching of the Twelve during our Lord's earthly ministry.
The so-called Grand Commission is, like I said, in the end of the three synoptic gospels:
18 And Jesus coming, spoke to [the Eleven], saying: All power is given to me in heaven and in earth. 19 Going therefore, teach ye all nations; baptizing them in the name of the Father, and of the Son, and of the Holy Ghost. 20 Teaching them to observe all things whatsoever I have commanded you: and behold I am with you all days, even to the consummation of the world. (Mt 28)15 And he said to [the Eleven]: Go ye into the whole world, and preach the gospel to every creature. 16 He that believeth and is baptized, shall be saved: but he that believeth not shall be condemned. (Mk 16)
47 ...penance and remission of sins should be preached in his name, unto all nations, beginning at Jerusalem. 48 And you are witnesses of these things. 49 And I send the promise of my Father upon you: but stay you in the city till you be endued with power from on high. (Lk 24)
The gospel of John records the same appearance in the Upper Room to the Eleven and includes the specific charge to them to either forgive or retain sins, briefly mentioned by St. Luke:
21 He said therefore to them again: Peace be to you. As the Father hath sent me, I also send you. 22 When he had said this, he breathed on them; and he said to them: Receive ye the Holy Ghost. 23 Whose sins you shall forgive, they are forgiven them; and whose sins you shall retain, they are retained. (Jn 20)
Your interpretation of what the Apostles were charged with is fantastical.