Ragesia: a rampant dragon wielding a torch.
Dassen: a book surrounded by 9 (or is it seven?) mountains, one for each of the domains in the union.
I don't know enough about Shahalesti to think of something good but since we're dealing with elves and a forested land, something with trees might be a good start. A star motif is also generic and works well with elves.
Pretty good ideas.
For Dassen, I would go with a central book, a mountain beneath it, and then four mountains on either side, like parentheses, representing the eight lands, and the central holding of the King (but the king is below the book, which represents their union).
The Shahalesti have a hard-on for the sun. I'd suggest something akin to the White Tree of Gondor in Lord of the Rings, with a sunburst behind the crown of the tree.
For Sindaire, they are a holding of Ragesia, but their original national flag might have been a two-tone background (blue on the left for the sea, gold on the right for the land) with a bear of the opposite color on each of the sides, and a diamond in each corner.
For Ostalin, I kinda like this early Mongolian flag, but probably with different colors. I'd give the background a dark stormcloud look, and trim it in gold. The central image would be a stylized mount - which could represent a horse, hippogriff, or, y'know, giant mile-long living airship troop carrier.
Dang. Now I kinda want to see this stuff in the compilation.