It's quite admirable how you manage to keep coherence between the different elements of the map.
Coherence also in your line, between things as different as illumination or a forest of fir trees....

The vignettes seem very clear to me and tell the story well. Not sure if you need to add text on the map to explain the differences of the regions as well, but it's up to you to see and find a solution that suits you between well understanding the actions of the dragons and not covering this (very) beautiful map too much with labels and other external elements.
Maybe one solution would be to put mentions like "hatching region" on the border? maybe just one word, very short? The diamond symbol should be enough to do the job of locating the regions, I think.
Both solutions seem to me to be quite appropriate.

For the positions of the forts, I think it's better this way... the use of natural barriers seems logical to me, and their arrangement remains harmonious, in my opinion.
The idea of the high plateau for the last fort also seems good to me, but if you cover this plateau with forest, you're going to have a lot of work to do... Maybe the river is more easy ?