"Fulfill all righteousness" is not about passing some test. It was about fulfilling the law in order to enter the priesthood. In Jesus' case, the Melchizedek priesthood. Remember, he came to fulfill the law.
In Exodus 29 and Leviticus 8 we learn about the ceremonial consecration to enter the priesthood. This included being washed by water, being annointed with oil (which represents the Holy Spirit), and being verbally blessed. The baptism of Jesus by John, who was a priest himself, the descending of the Holy Spirit, and the verbal blessing from God the Father, all followed the ceremonial blueprint for entering the priesthood. This gave him the validity in Jewish law and practice to teach and preach in the synagogues. His disciples called him "Rabbi".
Notice that this event is described in Mark 1, Matthew 3, Luke 3, and John 1, and what follows those passages is the start of Jesus' earthly ministry. He starts preaching the good news and calls disciples to follow him.
Jesus being baptized was not about him being "saved". Therefore, you cannot equate our human baptisms with salvation.
1) Did Jesus fulfill the law in order to enter the preisthood? Or did Jesus enter the priesthood and then fulfill the law? It has to be the latter because it was his event on the cross that finally fulfilled the law. He had the priesthood by that time.
2) To compare Jesus' baptism to some kind of entering the priesthood ceremony is a real stretch. The OT anointing of Aaron and his sons is far more elaborate and far more ceremonial than was Jesus's simple baptism by John. Jesus was not receiving the priesthood at the time of his baptism.
He was baptized, received the HS, and was praised by his God and Father.
3) I will give you points for coming up with a decent explanation about what 'fulfill all righteousness' meant. Did Strong give you the idea that it meant to fulfill the law?
But alas, Jesus's fulfilling of the law, which primarily refers to the final sacrifice of the law of Moses, which was himself. As he finally died on the cross, the law of Moses came to an end, and the law of Christ was opened. It was now up to his representatives, the apostles, to teach and preach the law of Christ and how it manifests itself in our life. In many ways the law of Christ released Christians from much of the burdens of the law of Moses. But in other respects the law of Christ was stricter than the law of Moses. For instance: in the law of Moses you would have to committ physical adultery in order to be charged with adultery. In the law of Christ, just lusting after women, even if you don't actually committ physical adultery, Christ says you have committed adultery in your heart. Much more strict.
4) finally, when Jesus made the statement, "for thus it becometh us to
fulfil all righteousness', he was standing with John the baptist on the banks of the river Jordan and was ready to be baptized. John seemed to hesitate and so Jesus helped him by saying these words. If Jesus had neant these words to fulfill the law, he would have said, "for thus it becometh us to fulfil the law'.
John was not going to be part of fulfilling the law, only Christ could do that on the cross. But John was part of the command to fulfill all righteousness.
Even Jesus needed a priest to help him with this sacred ordinance.