I think in many ways this whole discussion highlights examples of where exceptions prove the rule.
As for Arwen, she was both and Elf and a non-Elf.
Also, in comparison to Mithrellas and Melian, Arwen like Luthien had a choice. There was no point in Melian or even Mithrellas remaining in Middle-Earth forever. Arwen on the other hand would be as mortal as her spouse, and in actuality did not have the option of going to Valinor when Aragorn died (I rationalize his apparent remark to contrary elsewhere).
Moreover, I would argue that any such option effectively dissolved at the point of their marriage. Whereas for Elrohir and Elladan it is a healthy "who knows?"
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The hoes unrecked in the fields were flung, __ and fallen ladders in the long grass lay __ of the lush orchards; every tree there turned __ its tangled head and eyed them secretly, __ and the ears listened of the nodding grasses; __ though noontide glowed on land and leaf, __ their limbs were chilled.
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