Why does Virgil allow Juno to unite the Trojans and the Italians? Why not another character? How does this resolve the plot?
Troy was one of the greatest and wealthiest city-states in the ancient world. By claiming Troy as their place of origin, the Romans established a cultural and religious history for themselves. They were not a bunch of wandering nomads who happened upon a bare patch of land. They were the descendants of the great King Priam. Jane Gardener says of this connection:
It is not really surprising to find Romans willing to accept that their founders were foreigners. These stories enabled the Romans to claim their own place in the tradition, regarded as in a sense historical', of the Greek heroic past. (source linked below)
Why did Virgil choose Aeneas? Perhaps because there was already a tradition of his being one of the few survivors of the Trojan War.
How does this resolve the plot? After a few false starts and brief stays in other cities, Aeneas and his followers do finally end up in Italy and build their city. At the end, the Trojans assimilate with the local people. The eNotes summary of book 12 includes this comment:
Jupiter...[tells Juno] that she is no longer allowed to harass the Trojans....She only asks that after the Trojans unite with the Latins, that the Latins not adopt the Trojan ways and that the name of Troy die forever. Jupiter agrees, adding that the two races mingled will be known as Latins.