First, it's helpful to know the old-fashioned meaning of the English word, Testament. Throughout the centuries, a person with an estate (buildings, land, and/or possessions of some worth) would write out their "Last Will & Testament" which had to be witnessed and signed and dated by two people of sound mind, to testify that what the person had written was how he wished his goods to be divided up between various benefactors after his death, and that he was of sound mind when he wrote down his requirements. A person doing that was called 'the testator', had made a legal document that had to be carried out after his death. Nothing could be done until he had died, of course. That is a critically important point with regard to the Bible.
To begin with, there only were the manuscripts written in ancient Hebrew that pertained to the people of Israel, though the Gentile nations were written about a lot as well. That collection grew from the first writings (done by Moses, circa 1400 B.C. at the latest.) through various kings and prophets of Israel, the end of that growing collection being written by the prophet Malachi circa 433 B.C. The Jewish people who meticulously kept and copied those manuscripts did not call them "the Old Testament", but the collection was know as the Tanakh.
After Malachi, there was a gap of about 400 years before Jesus was born. After Jesus' death and resurrection, his followers finally understood the connection between the ancient Hebrew writings that foretold a Messiah, and Jesus being that one. His resurrection made everything clear. Some time after that, the apostles began to write the account of Jesus' life, death and resurrection (the gospel accounts), and there quickly followed a batch of other manuscripts (all in koine Greek) giving the story of the new Christian church and how it was to maintain the faith once delivered to them, plus containing prophecies for the future.
Once both sets of writings were put together (a necessity for Christians to understand just who their Christ was), the clear distinctions between the ancient times up to Malachi, and the new times from Christ's advent till near the end of the first century warranted classifying both sets of writings as we now have them - the Hebrew scriptures were called The Old Testament, and the Greek scriptures were called The New Testament. The date for that might have been after Marcion drew up his own truncated, anti-Jewish list of Christian Scripture (circa A.D. 201) for that was what prompted a final, settled canonical list of scripture.
Both sets (the Hebrew and the Greek manuscripts) were about God's legal requirements for his people, with God himself telling the Jewish people that he would make a new covenant (or, 'testament') which would include the Gentiles far more than ever the previous (old) covenant had done. The old was always stated by God to be that which would lead into the new. He had always made clear that no sinful human could ever keep the old law covenant, and breaking it would put those people under a curse. The Messiah came to redeem God's people from the curse of the Law by fulfilling it.
Once fulfilled, it had served its purpose, so Christ died, and that was when the terms of the new testament (covenant) started to take effect. Christ opened up an inheritance for Jews and Gentiles, through faith in his death. That is why the book of Hebrews says that "the law made nothing perfect, but the bringing in of a better hope did; by the which we draw nigh unto God... By so much was Jesus made a surety of a better testament" (Hebrews 7:19 & 22). This is where the necessity for Jesus' death and resurrection is spelled out for us:
"But now he hath obtained a more excellent ministry, by how much also
he is the mediator of a better covenant, which we established upon
better promises. For if that first covenant had been faultless, then
should no place have been sought for the second. For finding fault
with them, he saith, Behold, the days come, saith the Lord, when I
will make a new covenant with the house of Israel and with the house
of Judah: Not according to the covenant that I made with their fathers
in the day when I took them by the hand to lead them out of the land
of Egypt; because they continued not in my covenant, and I regarded
them not, saith the Lord.
For this is the covenant that I will make with the house of Israel
after those days, saith the Lord; I will put my laws into their mind,
and write them in their hearts: and I will be to them a God, and they
shall be to me a people: And they shall not teach every man his
neighbour, and every man his brother, saying, Know the Lord: for all
shall know me, from the least to the greatest.
For I will be merciful to their unrighteousness, and their sins and
their iniquities will I remember no more. In that he saith, A new
covenant, he hath made the first old. Now that which decayeth and
waxeth old is ready to vanish away...
How much more shall the blood of Christ, who through the eternal
Spirit offered himself without spot to God, purge your conscience from
dead works to serve the living God? And for this cause he is the
mediator of the new testament, that by means of death, for the
redemption of the transgressors under the first testament, they which
are called might receive the promise of eternal inheritance. For
where a testament is, there must also of necessity be the death of the
testator. For a testament is of force after men are dead: otherwise it
is of no strength at all while the testator liveth. Whereupon
neither the first testament was dedicated without blood.... and
without the shedding of blood there is no remission." Hebrews 8:6 to
9:22 A.V. (bold emphasis mine.)
This answer your first six specific questions; I don't know the answer to the seventh.