I didn't revisit the island immediately after conquering the the tower, but instead set out to explore the fjords. And later, when I did try to go back, the whole island was covered in poison fog. I've not been able to go there ever since, and I guess there would be little point in doing so anyway, since everyone on the island has likely been poisoned to death.
I have not completed the Boat-in-the-tree quest. Nor have I visited the wyverns' nesting place, since I thought it would be far more cool to go there later on, as a dragon ("Hello, small fry!").
