I don't think that Legolas was the Heir to the throne of Mirkwood. He was probably the second or third son, because those are usually the ones who get sent on the ambassadorial/diplomatic missions. The Heirs usually (key word, usually) stay home, and generally try to be safe. In the case of Oropher and Thranduil at the Last Alliance, they probably needed as many Elves as they could get. Other, the other possibility is that Thranduil went along with his older brother (who, obviously, was the Heir) and his father, and the brother and father got killed, leaving Thranduil the throne.
Most of the time, second and third sons are considered "expendable," which is why they're sent on missions. Granted, Legolas was just supposed to go to Imladris to give them the news of Gollum's escape, but a mission is still a mission, especially during times of war. Thranduil must have known that Orcs were concentrating around the Elven strongholds, and in general areas. Why would he have risked his Heir? The answer is, he wouldn't. He naturally would have sent one of his slightly younger, albeit experienced, sons instead.
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But Melkor also was there, and he came to the house of Fëanor, and there he slew Finwë King of the Noldor before his doors, and spilled the first blood in the Blessed Realm; for Finwë alone had not fled from the horror of the Dark.
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