Gandalf did die. Dead, dead, deadski. However, Gandalf in not a "man". He and the rest of the wizards were actually sent to Middle Earth at the end of the 2nd Age by the Powers That Be in the Blessed Realm of Valinor which lies to the West. Valinor is the original home of the elves before many of them were banished to Middle Earth. Gandalf, Saruman, and Radagast the Brown are more like spiritual Avatars created to counsel and guide the good people of Middle Earth. Their bodies can be killed and their spirits will return to Valinor. The Powers That Be basically slapped Gandalf's spirit into a new body and sent him back to complete his work in Middle Earth. That's why Gandalf seems confused when Aragorn et al call him "Gandalf". In the book it is much clearer that he is still trying to get his memories in order now that he is alive again.

The wizards are all given a color. Radagast was Brown. Gandalf was Grey. Other wizards would have been other colors. Saruman was their leader and, as a summation of all their colors, he was the White Wizard just as all colors in the spectrum are combined to produce white light. When Gandalf comes back as White, it shows that he is Saruman's replacement. Saruman has been cast out of the company of Wizards and Gandalf now leads them.

Because all the good accents were already used by everyone else. Bear in mind that American English is just a dialect of English just as Scottish or Jamaican or Indian English. And since when do American's begin every sentence with "M'lord?"

"Immortal" means "exempt from death". In the case of the elves that means, if you were to look at an elf's death certificate, you will never see the words "natural causes". They do not suffer death do to age, disease, infirmity, or being placed on their stomach in their cribs when they are babies. They can be poisoned, stabbed, burned, blown up, drowned, disemboweled, and bored to death just like any other living thing. When they are slain their spirits travel to the Halls of the Dead in Valinor where they just sort of hang around in Limbo. Men, on the other hand, die and go to... well, no one knows. It really freaks the elves out that mortal souls just sort of go someplace that even their gods don't know about. Of course it freaks the men out, too.

Why would Arwen endure the grief of losing Aragorn to old age? Children. Aragorn would be King of Gondor. King's tend to produce heirs. Arwen would not simply off herself for the same reason most of us would not kill ourselves when our husbands or wives die. Life goes on and her children, her grandchildren, and her great-grandchildren would still be a source of duty and joy to her. Besides, she would know she would eventually join Aragorn in death. He's not going anywhere so what's the hurry.

A little editorializing here because it is one aspect of the movie that continues to irk me. Elrond is called "Elrond Half-elven" in the books. That's because his parents were the human hero, Beren, and his mother was the elf maiden, Tinuviel. Beren and Tinuviel had several children and to each child was given the choice to be either mortal or immortal. Elrond chose to be an elf. His siblings chose to be human and mortal. Elrond's children can also chose to be mortal or immortal. If Arwen chooses to stay in Middle Earth with Aragorn she will be choosing a mortal's life; she will sacrifice her immortality. The movie is not doing a very good job of explaining exactly what she would be sacrificing if she were to wed Aragorn.

Saruman represents the coming of industrialization. Even before coal power and oil power there was water power. A dam represents a strong, nearby, easily controlled source of ready power. The dam, by the way, is a cinematic addition added to put in some cool CGI. In the book the ents just diverted the nearby river to flood the plain around Isengard. They effectively built a dam in the book; in the movie they tore down a dam. Saruman could justify having a dam because it supplied power and, from the point of view of staying true to the book, as long as Isengard is flooded, it really doesn't matter where the water comes from.