[Mithalwen]

I think in the notes to LACE Tolkien makes it clear that they regaraded it as a matter of honour not to present Thingol with a fait accompli..

Even though, as many of you know, I could hardly like Luthien less, she is less passive than Arwen. There is a sense that she carves her destiny while Arwen born in her image lets fate unwind. Leads to all sorts of questions (which I can not necessarily answer) about the workings of fate and destiny in the books since Eowyn who could be seen as a instrument of destiny withregard to the witch-king, has to defy the de-facto paternal authority over her to fulfil the prophecy whereas is like some cloistered heroine in a pre-raphaelite painting waiting for external events to decide her fate.