Trivia: Shortly after the battle begins, Bard rides to Dale to look for his children. A woman tells him she saw them in the old marketplace. When he starts to go there, a man comes running and shouts 'the Orcs have taken Stone Street'. That is an in-joke: Stone Street is the name of Peter Jackson's film studio.
Trivia: Fans may be unaware that Billy Boyd, who played Pippin in the Lord of the Rings movies, sings the end-credit song, "The Last Goodbye."
Trivia: This film has both the final acting role for Ian Holm, who played the older Bilbo, and the final live-action role for Christopher Lee, who played Saruman. (His only roles after were voice-overs.) Holm passed away in 2020, while Lee passed away in 2015.
Chosen answer: After the battle the Arkenstone was placed with Thorin in his tomb. It is shown that during the siege, prior to the battle, Thorin sends out a raven which returns shortly before the Dwarves arrive. This raven carried the message to ask for help. The mission to retake the mountain was not a secret among the Dwarves, Thorin had sought an army to help them from the beginning but only the few who went on the journey were willing.
Not quite, the mission to retake the mountain was organized by Gandalf and Thorin, who lived in exile with his family and small group of followers. Dain and other Dwarves did not know, as Gandalf never shared this information with anyone, not even Elrond. It was only after Thorin and Co. Actually arrived in Erebor that Thorin asked the ravens to travel to any Dwarf settlement nearby and gather the armies as the Elves and Men would no doubt come back to the Mountain. The raven was actually an old raven, so he only flew to the Iron Hills to get Dain.