Kam - you asked.
The King of Amroth's underlying motive was to further the war, as the book showed.
You didn't get to see what Rauven's stake was, since that would have overcomplicated a bit of the plot that was not relevant to the main story. But there is a story there.
On Talera's bridegift - you have mistakenly applied the term "elemental powers" to apply to the four elements, earth, air, fire and water. Light and Shadow do not fit into that catagory…nor should they.
Elemental power, as Asandir would define it, means that the powers the two princes possess draw directly OFF OF the elements, and in short, influence all of them, without boundaries.
So the power of Light and Shadow do not correlate to earth, air, fire and water, but use the forces of all elements.
What drives the powers of the elements - that would be source for the princes' abilities.
Hope this clarifies -
Sorry I took a bit to get back to you - I've been finalizing the black and white interior art for Fortress, and finishing production on the text. It's had me engrossed.