Idril like Thetis?

[Aiwendil]

Achilles' actions make sense as part of a soldier ethic-- I'm not sure how 'southern' that is-- could you elaborate on that?

Achilles's ethic (at least at the beginning of the Iliad) is typical of Homeric Greece - he is, of course, part of the Hellenic or 'southern' tradition (as opposed to the later Norse, 'northern' tradition). Yes, it is a soldier ethic, but it is a specific type of soldier ethic. For a Homeric Greek warrior, glory is the only goal worth pursuing. I'm not very familiar with Norse morality, but I'm sure it isn't exactly like the Homeric code. The northern tradition, I believe, emphasizes valor rather than glory; in this view no victory can last and there is a constant struggle against inevitable defeat.

At any rate, Tolkien was certainly much more influenced by Norse mythology than by Greek. Earendil is a very different character from Achilles and I think any parallels between them are an accident of the common themes used throughout mythology, not a conscious reflection.