Given that Jesus believed in the OT, and agreed with it, why was the intent not described or given back then?
I suggest it was, and that it is plainly obvious to anyone who thinks about it freely. But since the world is governed by the fear of having to protect ourselves from those who will use the law for personal gain rather than as it is intended, we have become conditioned to accept that letter-for-letter interpretation of law is the final matter. But, that is not the purpose of law. The purpose of law is to describe an intended judgement for certain situations. This is why law makers are very particular about the wording of new law, and why law is so heavily argued because when a legal ruling is given, the judges are bound by what the law says rather than what the law was meant to determine. Did you look at the situation in the field? Do you think that when God gave the commandment "You shall keep the Sabbath holy, on that day you shall do no work", He meant to say that nobody should be allowed to pick a fruit from a tree as they wander the garden with Him? Jesus said "If you had known what God wants, you would not condemn the innocent".
I think this is clearly the distinction that has been made between rulership of God's kingdom by men vs rulership of His kingdom by Jesus Christ:
“This is the covenant I will make with the people of Israel after that time,” declares the Lord. “I will put my law in their minds and write it on their hearts. I will be their God, and they will be my people.
.. whereas the kingdom that was instituted by Moses had been run by men, those who worshipped God according to His holy institution were subject to their rulership. Jesus has established a covenant with God whereby we can approach God in freedom and confidence, knowing that if we repent from our sin, He is faithful and just to forgive us our sin. In this way, the old covenant was a caretaker until Jesus came, to fulfil the law and to reconcile us to God, so we can worship Him
in spirit and
in truth. We do not answer to men who sanctify us, we answer to Jesus in spirit.
That gives us the freedom and confidence to know that while we are obeying His command, we are not subject to human interpretations of the law, and He is faithful to forgive us for our sin.
Also, was the intent of the Sabbath partly to have that specific day to rest - or can it now mean any day of the week?
You should ask Him what He requires of you in that regard. In this world today, you will be pressed to find any such day of rest. Even Christians who observe Sunday are obligated to attend church services, and look out if you are driving slow in front of them when they are running late! In the ancient Jewish society, it was a coordinated day of rest. There was no such thing as a sect of Jews who would rest on a Sunday. But that whole culture is out the window when you look at the world around us, but you know, if a church can get away with preaching that you must attend every Sunday because it is God's ordinance, that's pretty tempting.
(Why are there so many different interpretations if there is one Holy Spirit?)
We don't always listen to The Holy Spirit. If we did, we would agree, but it is hard. Have you ever butted heads on a public forum like this, and then had to take back your words? I have. It is really hard to do. Some people just cannot do it. Imagine though, someone who is recognised for owning a podium finding out they have been wrong! You know what St Paul said "it is by the grace of God that you have been saved, and not for any works that you can boast about".
Are you saying that to follow Jesus, you have to follow the intent of the original commandments given to Israel, in order to be saved, or do you just believe that Jesus died for your sins?
Jesus said this: "Not everyone who calls me 'Lord', 'Lord' will enter heaven, but only those who actually do the will of my father, who is in heaven". So while I am able to use scriptures to instruct you in righteousness and to correct you, really I cannot tell you what He requires of you. You will need to open the door, invite Him in for a meal, and find out for yourself whether He requires you to change. You need to do this often, BTW. Some people think this is a one-off thing, but I reckon every time you hear Him knocking, you should invite Him in. And perhaps reflect on how He got put outside last time.
If God doesn't want people to perish, then why does Jesus tell us the way is narrow and that few find it?
Most people do not really want to be corrected, they just want their desires met. Jesus said "work hard to enter the narrow gate". Mostly, the problems in our world are caused by us doing the wrong thing because we choose to please ourselves to that extent. It is a clear, consistent theme that we need to sacrifice ourselves for others. But most people really struggle with that. "It is easier for a camel to go through the eye of a needle than a rich man to enter heaven". We are all capable of being that rich man in some way, to hang on to something so badly that we will go away sad when Jesus tells us that is our stumbling block. In this man's case, it was wealth. In some people's case, it is reputation. Probably you get the idea.
I realise I've asked a lot of questions .....
Excellent questions, good asking too! I really hope my response has been sufficient to be helpful. Please pray to Him and ask Him to help you understand the truth, and think about why you would want to ask for that from Him before you do that. You seem like a person who would like to know the truth it and would make good use of it.