The point is you cannot keep the sabbath without keeping all of the Law.
“For the one who obeys the whole law but fails in one point has become guilty of all of it.” (James 2:10)
In James 2:1-10, he was not trying to warn them away from obeying all of God's laws, but rather he was encouraging them to repent and do a better job of obeying God's Law more consistently by not showing favoritism.
You are confusing the Ten Commandments (now abolished) with the Two Great Commandments that believers of all time lived by. And also live by today in the New Covenant.
Jesus was not asked about which of the commands should be followed, but about what the greatest command was. All of the 613 laws of the OT and 1,050 laws of the NT can be put into one of both of the categories of being about how to love God and our neighbor, so the greatest two commands don't replace the other commands, but rather they are the essence of the other commands.
They did not. They tried to kill Paul for teaching against it.
"Therefore let no one act as your judge in regard to food or drink or in respect to a festival or a new moon or a Sabbath day- things which are a mere shadow of what is to come; but the substance belongs to Christ." (Col 2:16)
If you look at Colossians 2:16 by itself, then it is ambiguous as to when Paul was saying not to let anyone judge them for keeping God's holy days or for not keeping them, but it we look at the context of the views of the people judging them and keep in mind the theme that we must obey God rather than man, then it becomes clear:
Colossians 2:8 See to it that no one takes you captive by philosophy and empty deceit, according to human tradition, according to the elemental spirits of the world, and not according to Christ.
Paul would never have described those who were teaching obedience to the holy, righteous, and good commands of God in accordance with the example that Christ set for his followers as taking people captive by philosophy and empty deceit according to human tradition and not according to Christ. He went into more details about what these elemental spirits of the world are later in the chapter:
Colossians 2:20-23 If with Christ you died to the elemental spirits of the world, why, as if you were still alive in the world, do you submit to regulations— 21 “Do not handle, Do not taste, Do not touch” 22 (referring to things that all perish as they are used)—according to human precepts and teachings? 23 These have indeed an appearance of wisdom in promoting self-made religion and asceticism and severity to the body, but they are of no value in stopping the indulgence of the flesh.
So the Colossians were keeping God's holy days in obedience to His commands and in accordance with the example Christ set for us to follow, they were being judged by those teaching human traditions and precepts, self-made religion, asceticism, and severity to the body, and Paul was writing to encourage them not to let any man judge them keep them from obeying God.
““Therefore I conclude that we should not cause extra difficulty for those among the Gentiles who are turning to God, but that we should write them a letter telling them to abstain from things defiled by idols and from sexual immorality and from what has been strangled and from blood.” (Acts 15:19–20)
Why no Sabbath?
When a employer hires a new employee, they don't want to make it too difficult for them by making them memorize everything that they will ever need to know about how to do their job up front, but rather they start with the basics with the understanding that they will learn how to do the rest on the job. Acts 15:19-21 expresses this same line of thought where they didn't want to make things too difficult for new believers coming to faith by making them memorize a long list of laws up front, but rather they started with the basics, which they excused by saying that they would continue to learn about how to obey Moses by hearing him taught every Sabbath in the synagogues, which implies that they were already keeping the Sabbath in obedience to God's command in accordance with what Jesus taught by word and by example.