These are impossible to answer totally. Each King Arthur story is different. Many are based on Mallory's "La Mort d' Artur," but, so many changes have been made to even that story (which was a Medieval romance, and not a history) that you need to know which book/movie you are talking about.
1,2 - depends on the book.
3, 4. - depends on the book. There was no Merlin in the earliest stories.
5. Arthur lives a LONG time before the middle ages! He was around just after the Romans left.
6. The real Arthur lived before feudalism was really established. the basic idea is that the King "owned" all the land. He would parcel it out to Vassals, who had to provide military service. These Vassals might have vassals of their own, going down the line.
7. Excalibur. There is much speculation on this - some thing it means "ex cali bur" or "from the river Cali," which is an actual place in England.
8. depends on the story
9. same
10. He had it made. I've seen it - in the Guildhall in Winchester. I took pictures of it.
11. This is in several stories. The idea was that only he was was destined to be King could pull the sword out. Only Arthur could.
25. This one I know a lot about, but, the story has been very warped. The Grail is actually "graal," which are cruets. Joseph of Arimathea (who took down Christ's body, and prepared it four burial) is said to have collected one graal of Christ's blood, and one of his sweat. Due to the persecutions of Christian Jews, he went to England, at Glastonbury. He founded a church there in 37AD. At some point, these 2 graals became lost. So, a noble deed would be to find them. But, somehow, in the stories the Grail became the communion cup of the Last Supper. I'm not sure when this happened. But, knights (no knights in the real Arthur's time, but...) on an honourable quest - VERY common in these sorts of adventure story.
26. Rebellion, an unfaithful wife, with his best friend, fights against evil... the real Arthur only had to worry about the invading English!
I've been fascinated with the story of the "real" Arthur for many years. Sadly, getting through all the fiction is a tricky business. We think of him as a Middle Ages King of all England. In fact, if he existed at all, he was a 5th century Romano-Celtic warlord, who fought AGAINST the English (i.e., the Anglo-Saxons). There were no castles, and knights and such in his day at all. His claim to fame was the use of horses in battle, which was not common then. Calvary usually beats infantry, especially an unprepared infantry. So, he won most of his battles (a dozen or so), but, was killed in his last one.