First off, Numbers 28 and 29 are dealing with sacrifice, I agree. The sacrificial system was done away with at the cross.
Deu 31:26 "Take this Book of the Law, and put it beside the ark of the covenant of the LORD your God, that it may be there as a witness against you;
Col 2:13 And you, being dead in your trespasses and the uncircumcision of your flesh, He has made alive together with Him, having forgiven you all trespasses,
Col 2:14 having wiped out the handwriting of requirements that was against us, which was contrary to us. And He has taken it out of the way, having nailed it to the cross.
This Book of the Law is the sacrificial system that pointed to Jesus coming and dying on the cross as the ultimate sacrifice for sin. This is completely and totally separate from God's 10 Commandments.
Deu 10:2 And I will write on the tablets the words that were on the first tablets, which you broke; and you shall put them in the ark.'
Deu 10:5 Then I turned and came down from the mountain, and put the tablets in the ark which I had made; and there they are, just as the LORD commanded me."
God wrote the 10 Commandments with His own finger on stone to show the importance of them. He asked Moses to write the ceremonial law because He knew that Jesus would one day die and the law of sacrifice would no longer be needed. The ceremonial law was also written on parchment of some kind, or else it couldn't have been "nailed" to the cross. You can't nail stone only paper.
Neh 9:14 You made known to them Your holy Sabbath, And commanded them precepts, statutes and laws, By the hand of Moses Your servant.
Two different "laws" spoken of here. "Your holy Sabbath" speaks of the 10 Commandments. "Precepts, statues, and law by the hand of Moses" is speaking of the law that was against us, the law that was nailed to the cross.
And to your claim that Adam and Eve didn't keep the Sabbath, I will have to admit that there is no verse stating that completely that I know of. However, Jesus created man on the sixth day and then rested on the seventh day. The story of the serpent and the eating of the forbidden fruit wouldn't have been the entrance of sin if they had not kept the Sabbath. The Bible is very plain that sin is the transgression of the law. So, by the very fact that they hadn't sinned until the serpent and the fruit story, they kept the Sabbath. Jesus also said that the Sabbath was made for man, not man for the Sabbath. (Mar 2:27) In other words, He could have rested on the sixth day and then created man on the seventh. But, man wouldn't have received the blessing of that first Sabbath with God. I would also like to add that after Jesus' creative work, He stated that everything was "very good." He did it right the first time, and after His perfect work, He created a memorial out of time for all to observe and remember what He did.
I will explain how to keep the Sabbath according to the Bible.
Exo 16:23 Then he said to them, "This is what the LORD has said: 'Tomorrow is a Sabbath rest, a holy Sabbath to the LORD. Bake what you will bake today, and boil what you will boil; and lay up for yourselves all that remains, to be kept until morning.' "
We should do all or baking and in-depth food preparation ahead of time so it is not done on the Sabbath. The key to keeping the Sabbath is doing everything possible ahead of time so it doesn't take time away from God. When you dated, you didn't wait until your date arrived to prepare the food or clean up the house. Your date would most likely take offense to that if it happened on a regular basis. The Sabbath was set aside for holy use.
Exo 31:14 You shall keep the Sabbath, therefore, for it is holy to you. Everyone who profanes it shall surely be put to death; for whoever does any work on it, that person shall be cut off from among his people.
Exo 31:15 Work shall be done for six days, but the seventh is the Sabbath of rest, holy to the LORD. Whoever does any work on the Sabbath day, he shall surely be put to death.
Exo 31:16 Therefore the children of Israel shall keep the Sabbath, to observe the Sabbath throughout their generations as a perpetual covenant.
Exo 31:17 It is a sign between Me and the children of Israel forever; for in six days the LORD made the heavens and the earth, and on the seventh day He rested and was refreshed.' "
Exo 31:18 And when He had made an end of speaking with him on Mount Sinai, He gave Moses two tablets of the Testimony, tablets of stone, written with the finger of God.
Exo 35:3 You shall kindle no fire throughout your dwellings on the Sabbath day."
God is not saying that you can't have a fire here. He is asking you not to start one after the Sabbath because of all the work involved in starting a fire. Cutting and splitting wood is very labor intensive.
Lev 16:31 It is a sabbath of solemn rest for you, and you shall afflict your souls. It is a statute forever.
The Sabbath is a time for meditation. We are to use this holy time to ask ourselves are we serving God with our life? Is our lives in harmony with Him. What choices can we make to better serve or Lord?
Deu 5:12 'Observe the Sabbath day, to keep it holy, as the LORD your God commanded you.
Deu 5:13 Six days you shall labor and do all your work,
Deu 5:14 but the seventh day is the Sabbath of the LORD your God. In it you shall do no work: you, nor your son, nor your daughter, nor your male servant, nor your female servant, nor your ox, nor your donkey, nor any of your cattle, nor your stranger who is within your gates, that your male servant and your female servant may rest as well as you.
Deu 5:15 And remember that you were a slave in the land of Egypt, and the LORD your God brought you out from there by a mighty hand and by an outstretched arm; therefore the LORD your God commanded you to keep the Sabbath day.
I can't speak for anyone else, but I was a slave to sin. God lifted me up out of my sinfulness and is showing me a new way to live. Because of what He has done for me, I am very thankful and willing to follow Him where He leads. I reached out and He lifted me up and by His grace, keeps me from the sinful things that I have been guilty of in the past. He is a mighty God!
Neh 10:31 if the peoples of the land brought wares or any grain to sell on the Sabbath day, we would not buy it from them on the Sabbath, or on a holy day; and we would forego the seventh year's produce and the exacting of every debt.
To keep the Sabbath, we should not go to stores and buy stuff. We had 6 other days to go to the store. Also, by going to the store or to a restaurant, we are causing others to break the Sabbath Commandment. If people wouldn't go to the store or restaurants, they would close down on that day.
Isa 58:13 "If you turn away your foot from the Sabbath, From doing your pleasure on My holy day, And call the Sabbath a delight, The holy day of the LORD honorable, And shall honor Him, not doing your own ways, Nor finding your own pleasure, Nor speaking your own words,
Isa 58:14 Then you shall delight yourself in the LORD; And I will cause you to ride on the high hills of the earth, And feed you with the heritage of Jacob your father. The mouth of the LORD has spoken."
The Sabbath was set aside from the very beginning of time to be holy time we spend with Him. Part of keeping the Sabbath is laying aside our desires to do our will and spend time with Him doing His will.
Isa 66:23 And it shall come to pass That from one New Moon to another, And from one Sabbath to another, All flesh shall come to worship before Me," says the LORD.
Proof that the Sabbath was not just for the Jews
Jer 17:21 Thus says the LORD: "Take heed to yourselves, and bear no burden on the Sabbath day, nor bring it in by the gates of Jerusalem;
Jer 17:22 nor carry a burden out of your houses on the Sabbath day, nor do any work, but hallow the Sabbath day, as I commanded your fathers.
Mat 12:12 Of how much more value then is a man than a sheep? Therefore it is lawful to do good on the Sabbath."
In one of the churches I attended, a group of people purchased gas on Friday and drove up and down I-75 on Saturday in the attempt to help stranded motorists. They would give them enough gas to get to a station, help change flat tires, or give them a ride if needed. This is definitely loving your neighbor as yourself!
Luk 4:16 So He came to Nazareth, where He had been brought up. And as His custom was, He went into the synagogue on the Sabbath day, and stood up to read.
We are to attend church on the Sabbath, that is also part of "keeping" the Sabbath.
As far as why do SDA's put such an importance on the Sabbath? The answer is quite simple. In Christianity, no one will argue about not killing, stealing, or bowing down to idols. In other words, of the 10 we see eye to eye on the other 9. So, it is logical that if a portion of Christianity is not observing all 10, then the one that is not being observed is the one that needs attention. If you were observing the Sabbath, but committing adultery, I wouldn't be writing all of this. I would be reasoning with other verses to attempt to show you to keep the seventh commandment.