I don't understand one thing. How does Legolas' heritage affect his departal from ME? He doesn't need to be one of the Eldar to leave, or to hear the call of the Sea (speaking of that one, Men also hear it, so why can's moriquendi?).
His background wasn't finished (according to the
Enc. of Arda, there are two versions - one that says Oropher is a Sinda who came from across the River, and another that he was a Silvan Elf), so there's no point in trying to prove any story.