The only elves leaving are the "High" elves. These were the Noldor who had travelled to the land of the gods in the west in an earlier age and had returned to Middle Earth to fight Morkoth - Sauron's boss. In the end of that war the gods returned to Middle Earth, defeeated Morkoth and "bent" the world so only the Noldor could return to Valinor - the land of the gods. The elves are immortal and after a time tire of Middle Earth and yearn to return to Valinor. This is the leave-taking they talk about. The reason they are all going to leave now is that the next age will be dominated by Sauron or Men leaving no room for the remaining Noldor. As a side note Legolas is a Wood elf and is not entitled to go to Valinor.
Orcs/goblins are interchangeable in the books.
Arwen is special. She is the daughter of Elrond Half-Elven. Elrond's mother was a Noldor and his father was human. His brother chose mortality and founded the kingdom of Numenor - the land the Gondorians came from. Elrond chose his Elven heritage and is immortal. Arwen has the same choice to make. She can chose Elven and go to the West or she can chose mortality and "become" human. The necklace is a symbol of her decision to want to be human and stay with Aragorn.
Orcs do not turn to stone. Only Trolls do. Orcs however fear daylight and do not fight effectively in it.
Boromir was not a gimp. He was a noble of Gondor, son of the Steward. His pride in Gondor and its people and its needs blinded him to the peril of using the ring. He fell under its spell and tried to take it from Frodo. He redeemed himself defending the hobbits from the Orcs and died nobly at the end of FOTR.