1. Question Restatements :
- Why didn't the Pharisees charge Jesus against his claims apparently "changing" Mosaic law?
- Why didn't Jesus make a much stronger argument if amending/abrogating Mosaic Law?
2. Jesus Actually Was Prosecuted for "Inciting Rebellion" :
Actually, Jesus was accused of inciting rebellion - and Jesus even admitted to claiming to be the "King of the Jews", (before the Sanhedrin and perhaps Pilate) - yet Pilate declared him guiltless of Rebellion, (likely against Caesar).
In later Judaism, it is documented that there was/is an offense regarding a Rebellious Elder based on Deuteronomy 7:12 who does not acknowledge the authority of the Oral Law, (i.e., Pharasaic Authority. Mishneh Torah, Mamrim 3:1). See also related Question, at Judaism.SE : Mamrim 3:1 - Who is the Rebellious Elder Mentioned in the Torah?.
NASB, Luke 23:1-3 - Then the whole body of them got up and brought Him before Pilate. 2 And they began to accuse Him, saying, “We found this man misleading our nation and forbidding to pay taxes to Caesar, and saying that He Himself is Christ, a King.” 3 So Pilate asked Him, saying, “Are You the King of the Jews?” And He answered him and said, “It is as you say.”
NASB, Luke 23:14 - and said to them, “You brought this man to me as one who incites the people to rebellion, and behold, having examined Him before you, I have found no guilt in this man regarding the charges which you make against Him.
The Sanhedrin was afraid of the "Multitude" :
NASB, Mark 11:32 - But shall we say, ‘From men’?”—they were afraid of the people, for everyone considered John to have been a real prophet.
They didn't agree how to accuse Jesus - Judaism was extremely divided during the Second Temple Period :
Several different accusations were raised, during Jesus' trial, (See Matthew 26).
It wasn't "just the Pharisees" : there were the Essenes, the Sadducees, Beit Shammai, Beit Hillel, etc. For example, the "Oral Law" was considered blasphemous, and influenced by the Pagans of Babylon - by the Sadducees, etc.
A passage in Acts illustrates the severity of the divisions :
NASB, Acts 23:6 - But perceiving that one group were Sadducees and the other Pharisees, Paul began crying out in the Council, “Brethren, I am a Pharisee, a son of Pharisees; I am on trial for the hope and resurrection of the dead!” 7 As he said this, there occurred a dissension between the Pharisees and Sadducees, and the assembly was divided. ... 10 And as a great dissension was developing, the commander was afraid Paul would be torn to pieces ...
3. Why didn't Jesus make a stronger argument to justify changing Mosaic Law?
Jesus believed people wouldn't believe - no matter how/if he explained:
The text states that following their rejection of Jesus, (John 15:26), Jesus delegated explanations, and further revelation, by promising to send the Holy Spirit, (John 16:13) - who would bring them to remember, (John 14:26), the impossibly enormous amount of information he taught, (John 21:25), and explained.
NASB, Luke, 16:30 - But he said, ‘No, father Abraham, but if someone goes to them from the dead, they will repent!’ 31 But he said to him, ‘If they do not listen to Moses and the Prophets, they will not be persuaded even if someone rises from the dead.’”
NASB, John 5:46 - For if you believed Moses, you would believe Me, for he wrote about Me.
Jesus, repeatedly and explicitly, refused to explain things that he felt had been explained before:
NASB, Matthew 9:13 - But go and learn what this means: ‘I desire compassion, and not sacrifice,’ for I did not come to call the righteous, but sinners.”
NASB, Luke 24:25 - And He said to them, “O foolish men and slow of heart to believe in all that the prophets have spoken!
NASB, Psalm 78:2 - I will open my mouth in a parable; I will utter dark sayings of old, (Matthew 13:35).
NASB, Galatians 3:8 - The Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, saying, “All the nations will be blessed in you.”
NASB, 2 Peter 3:2 - that you should remember the words spoken beforehand by the holy prophets and the commandment of the Lord and Savior spoken by your apostles.
Detailed explanations may have been omitted - for brevity, (manuscript size) :
NASB, John 21:25 - ... I suppose that even the world itself would not contain the books that would be written.
This question is similar to, "Why isn't the Emmaus Road Sermon actually in Scripture?"
NASB, Luke 24:25-27 - And He said to them, “O foolish men and slow of heart to believe in all that the prophets have spoken! 26 Was it not necessary for the Christ to suffer these things and to enter into His glory?” 27 Then beginning with Moses and with all the prophets, He explained to them the things concerning Himself in all the Scriptures.
This question is also similar to asking about Paul's Missing Sermons :
NASB, Acts 9:22 - But Saul kept increasing in strength and confounding the Jews who lived at Damascus by proving that this Jesus is the Christ.
4. Conclusion, The New Covenant :
Regardless of the reasons - it is consistent in Scripture to assert that Jesus anticipated that the Holy Spirit would fill in the gaps, (Jeremiah 31:33).