I haven't gotten to exploring the whole Island yet, but are these the "only" settlements?

-Raven Rock

-Tel Mythrin

-Skaal Village

-Thirsk Mead Hall

-the nord camp (before retaking Thirsk)

Unless you count the werewolf pack. Yes.

What about Castle Kaarstag? Is it populated by (hostile) rieklings?

Not that I know of, it's housing something else.

Lore-wise, a Dunmeri city named Oleer Mar (maybe that's the Dunmer name for Raven Rock) and a Dunmeri village named Sathil should be around somewhere, but the novels were disregarded, it seems.
Now you can imagine those places are still around, but you just don't encounter them ingame for obvious limitations of the DLC, or you can make up some story that those places were destroyed in the timeframe between the novels and Dragonborn.

Thanks for not spoiling it. Now I'm intrigued..